Weekly Questions and Answers
Questions for this column are submitted by powder coaters just like you who are seeking ways to improve efficiencies and solve every day problems on their powder coating lines.
Click on questions below to view answers.
Q: Hello, I would like to know if the powder coating process induces a magnetic field in ferrous sheets and tubes. Could you provide any specific detail if so. Thanks, J.B., Austin, Tex.
No. Magnetism isn’t part of the powder coating process. If your metal has magnetic properties, it got it from somewhere else; not the powder coating system. —N.L.
Q: I’m having cast aluminum parts powder coated in China with a medium texture flat black powder. When I receive the parts, the powder coating can be scraped off, and it’s being chipped during shipment. What’s the cause of this? The coating should be more durable than this, right? Another question: Is there a flat white powder available to withstand 572°F (300°C) to 662°F (350°C)? J.E., Little Neck, N.Y.
My, my, when will industry in this country learn to keep products here, in part for good quality control? How the heck are you going to make sure you have properly coated parts thousands of miles away? OK, so much for the sermon. The issue is the proper treatment of the aluminum before powder coating. If you did not specify all of the coating conditions, then you’re getting what you asked for. If you did, someone gets a trip to China! Aluminum must be cleaned and etched to remove the self-protective coating (patina) that in hibits the adhesion of the powder coating. If the coater is cleaning with an iron phosphate with no fluoride additive or conversion coat, then the coater isn’t removing this protective coat. As a result, the aluminum will shed the powder coating. And be aware that cleaning the parts and then letting them sit around for days will only allow the aluminum to try to protect itself again—thus a second cleaning. A crosshatch test would be most helpful with samples run and sent to you with each batch of parts coated. Make them do this ALWAYS. —G.T.
Q: Do you have a suggestion for powder coating the inside of pipe that is of various diameters and lengths? A customer wants to use a wet primer and then powder. What type of equipment is needed? R.C., Shoshoni, Wy.
Interior pipe coatings has been applied with powder coating equipment going back to the late 1970s. The minimum inside pipe diameter is 6 inches. Lengths in excess of 20 feet have been routinely powder-coated. Specialized equipment is necessary to perform this work, specifically pipe coating systems. These systems use collectors at each end of the pipe, and the gun(s) are fed through the pipe on a lance. A wheel is lo – cated at the bottom of the lance to support the lance and gun weight when inside the pipe. With the powder turned off, the gun is inserted into the pipe at a speed of up to 100 feet per minute. The gun is then actuated on and coats the pipe on the way out as the gun is extracted at a much slower speed. The collectors at each end of the pipe collect the overspray (the actual booth cabin is the pipe itself). Often several pipes are coated in parallel with multiple gun and lance systems. During the late 1970s and early 1980s, many of these systems were sold to pipe manufacturers supplying the drilling industry, although they are currently not as popular. You can purchase this type of specialized equipment from any of the mainstream powder coating equipment suppliers, if they still make them. —N.L.
Q: I’m looking for a powder coatings manufacturer who has a highly fuel resistant powder. Do you know of any company? B.H., Spring Lake, Mich.
There are only so many resin and additive combinations available to the formulator, so most powder producers will have something in their bag that should work. Epoxies have excellent chemical resistance, and a hybrid material will give you good chemical resistance and yet have ultraviolet-light resistance. I don’t know what your application is, but most fuels evaporate long before they do any damage to the coating. Vehicles are powder-coated with polyesters and acrylics with no effects at the fuel filler area. Now, if you want to immerse the fuel in a powder-coated vessel, that’s another matter. Then you need to test a good grade of epoxy. —G.T.
Q: We are into sheet metal fabrication with powder coating. For some days now, we have been facing pinhole problems in our powder coating. The diameter of the pinhole that appears on the powder-coated components starts from 0.2 micrometer in diameter to 1.0 micrometer in diameter. We have a nine-tank pretreatment process (hot process) before going to powder coating. To remove the moisture in the air, we have provided a dryer, a moisture separator, an oil separator, and an automatic drain valve. We are not able to eliminate the problem. We are looking forward to your suggestions. J.C., Coimbatore, Tamil Nadu, India
Pinholes in powder coatings are created when a gas escapes through the coating when the coating surface hardens, leaving a hole. Earlier in the cure cycle when the coating is still in its liquid state, the coating will reflow over the affected area, and there is no visible problem. Now all you have to do is figure out what is causing the release of the gas that is piercing the coating. If your substrate material is a casting, it’s likely that the gas is entrapped within the metal and escapes under heat. In this case, preheating the casting can help to drive the gas out of the metal before you coat it. If your substrate is a weldment and the pinhole is in the weld area, porosity can be your problem. In this case, the weld has holes that can trap moisture or contaminants that vaporize in the cure oven. The resultant gas causes the pinhole. Poorly cleaned or pretreated surfaces can cause pin holes because contaminants left on the surface under the coating will vaporize and pierce the coating surface. Sometimes contaminants can be introduced after the cleaning/ pretreatment process. Mishandling the parts, airborne contaminants in the plant environment, oil or moisture in the compressed air, and so on are all likely sources of these post-pretreatment contaminants. The worst airborne contaminant is silicone, but this usually causes a defect called a fisheye, not a pinhole. From your question, it looks as if you have enough cleaning stages, a compressed air separator, and your substrate is sheet metal. Therefore, you can eliminate outgassing from castings. However, all the other scenarios are still in play because your nine-stage cleaning/pretreatment system or compressed air separator may not be functioning properly or providing a water-break-free surface on the substrate. Don’t forget the airborne contaminants in your search to isolate this problem. —N.L.
Q: Our two powder coating processes are batch done in a nonreclaim booth and an automated booth. The automated line usually runs at about 7 feet per minute with the rack hooks spaced out at 2 feet. We were considering automating the batch booth process by using operators to spray the parts going past. What line speed would you recommend for this? On average, it will take our skilled operator 51 seconds to coat 12 of our hardest parts on a rack. K.K., Jamestown, N.Y.
Usually, and I underscore usually, the volume of product off of the line dictates the line speed. With existing equipment, the oven or the washer times will be the determining factors, not what a person can spray. Having said that, you can get ridiculous and set a line speed too fast for one operator to be able to properly coat the parts going past. In that case, you add another operator. Part design and coating requirements enter into the formula as well. Anyway, don’t start with what the coater can do. I need to know a lot more about the system and the parts you are coating to arrive at any line speed. —G.T.
Q: My question is about color changes. We apply white triglycidyl isocyanurate (TGIC) and low-gloss black powders in the same booth. We reclaim. After changing from white to black, we see cross contamination as little white specs in the black. When we change from black to white, we see no contamination. We clean the booth the same for both white and black. We take about 1.5 to 2 hours per changeover. This is done three times in 24 hours. The only thing that we reuse is the booth and the cyclone. These are completely cleaned out by washing and wiping them down. The powder hoses and the reclaim hoppers are only used for their designated color. Is there any more that we could do to stop the white in the black? J.M., Adrian, Mich.
Is the white a low gloss or a high gloss? Offhand, I’m not sure what the gloss has to do with it, but the white, if it’s a higher gloss, may be at least part of the problem. However, this says that there is certainly some very minor detail that’s different in the cleaning process. There may be some eddy currents in the airflow that are trapping some powder, but I don’t know why it would trap one color versus another. I would have to have more details on the cleaning process and the equipment powder path to be of any assistance. —G.T.
Q: Is there any such thing as a no-bake powder coating? If so, do you have any details on it? Thank you for your response. B.S., Taylor, Mich.
All powder coatings require heat to melt and flow to form the coating on the part surface. Some specialty powder coatings can be melted and cured at very low temperatures (around 250°F). Others, referred to as ultraviolet-curable (UV-curable) powders, are melted with heat at 250°F and then cured by using a UV light source. Finally, there are powder coatings that can be applied by using a flame-spray gun that ignites liquid propane (LP) gas at the gun tip, which melts the powder as it’s applied to the intended surface. All these methods involve heat to some degree, but offer alternatives to baking the powder coating onto the part. —N.L.
Q: If you had to decide between liquid or powder painting, what would be your decision and why? R.S., Distrito Federal, Mexico
It depends on what you want the coating to do and what your existing painting capabilities are. From a pure coatings standpoint, powder will be a better coating. However, other factors influence the choice, some that you can accommodate and some that you can’t or won’t.
There are several books written on the subject. Check the online Article Index sections on this website. The Powder Coating Institute also offers a text on the subject, visit www.powdercoating.org to search for it. You didn’t give any other details, so I can’t get too far with helping you. To repeat, when it comes to the coating, powder would be my choice over liquid. —G.T.
Q: How can we control powder consumption? Do you have any tips or methods we can use? S.B., Noida, Uttar Pradesh, India
This question begs a dissertation! So many factors are involved, and I don’t have much information to go on, so I can only respond in general terms.
Powder consumption can be high because of too much film build on the parts you’re coating. Consumption can be high because of poor hanger design. It can be high because the parts have been designed for something other than a spray application. If you have the correct booth, with a reclaim system, you should be recovering the overspray and blending it with virgin material. If you have too much overspray, eventually the reclaim will be mostly fine particles, and you’ll have problems with fluidization, application, and so on.
Here are some things to check: Is the airflow in the booth such that it contains powder, or does powder escape out of the openings? Is the hanger design so poor that the part surface isn’t presented properly to the spray gun? Poor hanger design will cause too much overspray, and you’ll likely have too much coating on the hanger. Make a list of all the things to look for, and then check them out and systematically mark them off of the list until you find the problem. Then fix it. —G.T.
Q: I was recently asked how to apply two-tone powder coating to a wheel. Could you please inform me on how to address this issue. I’m kind of baffled. Will the spokes need to be removed? I’ll wait to hear back from you. Have a blessed day! L.J., Spring, Tex.
Thanks for the blessing. I can always use an extra one now and then. You have two choices when powder coating a spoked motorcycle wheel in two colors. First, you can disassemble the wheel, coat the parts separately, and reassemble it afterwards (as you state in your question). This is the most bulletproof way of obtaining what you want, at a higher labor cost for the disassembly and re assembly of the wheel. The second choice is to mask the wheel and coat it assembled. This would require you to apply the first coat and only partially cure it for handling. Then you would remove the masking and remask the wheel covering that you just coated, exposing the new area. Now you will fully cure the wheel to ensure full coating properties. Be aware that the masking line where the two coats meet will be very sharp, requiring some final work (wet sanding and polishing) to blend the two areas. Good luck. —N.L.
Q: I have coated several items with high-gloss clear. The clear looked great for the first couple of weeks. Now the clear is cracking in spots. I have coated two items with the same clear within 24 hours, and both items have cracked in spots. What am I doing wrong? Hope you can help with my problem. C.L., Birmingham, Ala.
Is the clear coating material coordinated with the basecoat below it? You can’t just arbitrarily apply a clear over any old basecoat. The two need to be compatible. Check with the people who supplied the powder to you. —G.T.
Q: We have powder coating booths, and I was wondering what the regulations are to clean powder from employees in Michigan. Can you use air to blow off powder? N.H., Grand Rapids, Mich.
I have no idea what the rules are in Michigan. However, the Federal OSHA (Occupational Safety and Health Administration) specifies that compressed air must be regulated, dispersed using safety nozzles, or both to ensure that full line pressure cannot be used to clean operators. However, even with these precautions, there will be sufficient air pressure to remove most powder from an operator’s clothing.
For the long-term wellbeing of your operators, you should look into using Tyvek suits (overalls) that are cheap and disposable. This will help you in two ways: It’ll keep your operators’ clothing clean, and it’ll eliminate contaminants from their clothing (lint, dirt, etc.), which can adversely affect your process. —N.L.
Q: I build large aluminum globes of the world and I have a 5-foot-diameter globe that we need to first powder coat with clear and then have the outsides of the continents painted to resemble the ‘Blue Marble’ NASA image from space. My questions would be: Beyond scuffing, what other prep do I need to do? Is there a powder type/primer chemistry that works well together? Do I need a primer? My artist will be mixing paints on a palette for the topcoat, so she can’t use high VOC or two-part paints. M.B., Chicago, Ill.
I have to say that this is one of the more unusual questions I received lately. Having said that, I think I can help you. If these globes are used indoors, a primer is unnecessary for good product life. A standard polyester powder coating formula will provide great service life during the use of the product.
Scuffing the surface to ensure good intercoat adhesion is important before you do the artistry. For extra measure, a clear topcoat using normal liquid technologies will encapsulate and protect the artwork. —N.L.
Q: I am very accustomed to working with robotic spray and dispense applications in manufacturing. However, with powder coating applications, I have only seen manual and vertical reciprocator methods used. Does anyone use robots with powder coating applications, and what are the pros and cons of using a robot? S.J., Des Plaines, Ill.
Powder application has been done with painting robots for many years. In the early ’80s, I installed a robot with a standard powder gun to coat the inside of a box. It worked very well. The company did that for quite a few years before the product line was moved elsewhere. Several of my employer’s competitors were doing the same thing. The automotive industry is doing this with a two-headed application gun for certain areas on new car bodies. Several companies have used powder bells mounted on a robot to coat their products. So, there are powder applications with robots. Probably the biggest problem with robotic applications is protecting the working parts or the robotic arm so that the powder doesn’t get into some very intricate moving section, such as the robotic wrist. The service life of a robotic wrist will be very short if exposed to the powder. The robotics companies make covers for the delicate parts of their products. The big benefit with using robots is that they can coat areas that a person would complain about, and the robots don’t have to take breaks.
This isn’t meant to be a criticism of manual spraying, but product designers don’t always consider what has to be done to get a part coated. When you require a person to put on a complete suit, with mask and gloves, and tape the sleeves, and then the overspray completely covers the person in a matter of minutes, you’re asking a bit much. Try doing that 8 hours a day, 5 days a week. This is the kind of job you give to a robot. I won’t even get into consistency of finish and operator fatigue! —G.T.
Q: One of my customers asked if he could overcoat fusion-bonded epoxy. The material is applied electrostatically in a spray booth. We apply the epoxy to AWWA (American Water Works Association) C213 standards. We manufacture carbon and stainless steel pipe couplings for the water and wastewater industry. They are from a little less than 3 inches to 240 inches in diameter. T.P., Doraville, Ga.
By overcoat do you mean recoat? You should be able to do this, but be aware of any tight tolerances and the possibility that the heavy coating might cause some fit problems. You’ll want to check for inner-coat adhesion on a regular basis, and you’ll want to be sure that the parts are clean before doing any overcoating. A good wash job will do; pretreatment chemicals are unnecessary. How is the adhesion on the stainless? —G.T.
Q: What type of powder coating is best for exterior railing or ornamental iron work? A.L., Redwood City, Calif.
When I specified my railing coating requirements, I selected the following: Blasted steel, iron phosphate pretreatment, zinc rich epoxy primer, and TGIC-based polyester topcoat. This painting specification is easy for most powder coaters to meet. The only improvement I would have made was to add galvanizing before iron phosphate, but most local coaters can’t do this. I hope to never paint my railing again in my lifetime. —N.L.
Q: Is there a process for removing the powder from silicone plugs and caps? We’re getting a buildup, which is causing a bridging effect when we reuse the plugs too many times. We roll the plugs in our hands and try to crack the powder so it falls off, which works to a point but is a tedious process. Any suggestions would really help. D.A., Ivyland, Pa.
There seems to be some deep dark secrets about how to clean and reuse silicone masking devices. There are companies that will do it for you, at a cost of course. And I know of companies that soak the plugs in a mild solvent overnight and then blow them off the next morning. I know that when you talk with a company that has a method, they guard the method as if it were gold. And one of the last people they would tell is a columnist! Anyway, give the solvent method a try if you haven’t already. Start with a very mild solvent and work your way up the chain, keeping all safety precautions in mind. —G.T.
Q: What causes starbursts in wrinkle powders? D.R., Asheboro, N.C.
Starbursts, or at least the common use of the term, means kilovolt (kV) rejection. This is caused by the gun being too close to the part or having the kV setting too high for what you’re spraying. Starbursts may also be caused by the use of magnetic hang points, but so few people use magnetic hang methods that this would be a very rare. If these situations don’t apply, the problem is in the powder; but I really have my doubts about that. You’d have to have an epidemic of starbursts all over every part. The solution for that is to change powders. If nothing I’ve discussed fits your problem, then you’ll need to give me a better description of what your “starbursts” look like. —G.T.
Q: I have some powder left from three years ago. As long as it sprays, will there be any curing problems? M.H., Leola, Pa.
Toss this old powder. You definitely don’t want to use it. Who knows what you might end up with! —N.L.
Q: I have an issue with bleed-through when using an industrial paint pen. It’s a yellow opaque ink. The application is to mark product by customer order and lay out hole locations on roll-formed and structural angle. The bleed-through happens on our light colors. Is there a product that will survive the outside elements and multiple handling but will wash off in our processor’s rinse tanks, which are industry-standard processes. Thank you in advance for any possible solutions or products to test. D.C., Dalton, Ohio
I don’t know of a marker that will remain on the painted surface, withstand some abuse, and be removable in the rinse tanks. I would suggest using very thin colored plastic “dots” or some similar marker that stays on a product pretty well but also can be removed. Some of these markers are used as masks and can withstand cure temperatures without leaving any residue or defects. Masking companies are listed under Masking Products in the magazine’s Online Buyers Guide. —G.T.
Q: Could you provide information on primers and abrasion techniques that could be used to improve adhesion of powder coating to electroless-nickel (e-nickel)? Our customer wants painted e-nickel because of the electromagnetic interference (EMI) characteristics of the e-nickel coating. The customer wants powder coating because of the added aesthetic value. Any information you could provide would be helpful in solving some of our current problems with coating e-nickel. E.H., Cedar Rapids, Iowa
Whatever you do will add some expense to the process. The easiest way to improve adhesion would be to lightly abrade the nickel. This would have to be tested to assure that you haven’t negatively affected the nickel. You could apply a primer, but this doubles the process as you well know. I would hold that idea in reserve in case the abrasive method doesn’t work. After all, you only need to present a slightly rough surface to the paint for it to adhere to the substrate, so don’t get carried away. As you probably guessed, a primer will adhere somewhat to the nickel but may not be to spec, so I suspect that the abrasion method is your best bet. Try an abrasive pad first. They come in various grit sizes, and you can get most of them for manual use or affixed to a sander. —G.T.
Q: Is e-coating (electro-deposition coating) similar to powder coating? If different, what’s different about the applications? Is there anything different between the two applications with the Faraday cage? R.L., Oriskany, N.Y.
The only similarities between e-coating and powder coating is the word coating. I assume you know what powder coating is, and I won’t bore our readers with information they already know about the finishing process that’s the very name of this magazine.
E-coating is an immersion finishing process in which parts are dipped into a tank of paint (often epoxy), and an electrical charge is sent through the paint to attract the paint to the part. The part may have a negative or positive polarity, depending on if the e-coat tank is anodic or cathodic. The coatings are limited in formulation and color for this process, unlike powder coatings, which come in numerous formulas and colors for just about any application.
E-coat is applied in very thin films (often around 0.2 mil). It can be used as an inexpensive topcoat but is most often used as a primer. There are no Faraday cage issues with e-coat, but there are limitations to where the coating can be deposited. These areas are defined as low current density areas and areas where “throw power” limits coverage. This limitation is significantly less of an issue in e-coating than the Faraday coverage issue in powder coatings.
Finally, e-coating can be applied very inexpensively. If the average applied material cost for powder coating is $0.15 per square foot, the average applied material cost for e-coat is $0.02 per square foot. —N.L.
Q: Can powder coating replace nickel plating? We already have the equipment to apply powder coating. We’re having problems with corrosion (rust) in nickel-plated parts (housing bearings) during overseas shipments to a customer. That’s why we think we can replace the nickel plating with powder coating. The parts are sent to a vendor to be nickel-plated, but we’re having problems with the coating. The vendor is probably not doing it right, so we thought we could use powder coating to protect the parts better. The housings are made of carbon steel. C.B., El Salto, Jalisco, Mexico
Well, you can, but it would depend on what the coating is supposed to do. I would need to know what the coating performance specifications are to pass any type of judgment. Powder would certainly be a cheaper coating, and in this country nickel presents environmental problems. Nickel is certainly a hard surface coating, and tends to be a heavier coating than powder coating, so part tolerances would be an issue as well. I certainly wouldn’t make a change until I understood the coating performance specifications. By the way, there are ways to protect metal parts that are shipped via boats to prevent oxidation. But based on the information you’ve told me, powder would do nicely if the film build doesn’t create any problems. I suspect that the film thickness will have to be 2.0 to 2.5 mils. If that works for you, then go for it. —G.T.
Q: We currently powder coat our aluminum brake calipers red and black. They frequently see temperatures around 300°F and are subjected to hot DOT 3 brake fluid. We’re using bright red polyurethane and black polyester TGIC. We apply the colors separately and cure them at 400°F for 40 minutes. After they’ve been cured, we do a hot pot test where we submerge the caliper in hot DOT 3 brake fluid at 250°F for 2 hours, remove, and let cool, wiping off excess brake fluid. We then bake at 375°F for 20 minutes. After that time, the red is still rock hard, and the black is soft and tacky. What would cause that? C.S., Camarillo, Calif.
There is some speculation on my part, but I would think that calipers of aluminum would reach cure temperature in less than 15 minutes. I don’t know how much less, but less. That would mean for instance that the powder cure is ±30 minutes at 400°F. You should check your powder cure schedule. I think you’re over-baking, and that may be the problem with the black. You need to get your hands on a temperature recorder for the oven, and rather quickly. Your powder supplier should have one that you can use for a brief time to develop a cure chart. Without one, you’re groping in the dark. You’ll need it for about a day. Run more than one test. These instruments fit into a heat-resistant thermal box. Depending on the type of unit, it may have from three to six or more probes. The probes measure metal temperature on several locations on the part, usually a high point and a low point, and if so equipped, a mid-point. Another probe measures oven air. These instruments are critical in detailing information about what’s going on in an oven. You may have hot or cold spots due to improper adjustment of the dampers. The thermocouples may be bad or failing and giving false information. You get the point. Typically, the instrument is encased inside a thermal protective unit and has a graph ribbon on which the various probes will make their color-coded lines. The probes are usually numbered so that you can tell which line is air temperature and which lines are metal temperature. In theory, you should have a nice ramp-up set of lines for metal temperature. The air reading will go up much quicker, and the lines should be fairly straight to the exit point. If your production levels are at least moderate, and you’re planning to grow your business, then you should consider purchasing a temperature recorder for your oven. Frankly, I don’t know how you could operate without one. —G.T.
Q: We’re somewhat new to powder. We’re having issues with clumping due to humidity here in Indiana. We don’t always have our materials stored in our air-conditioned areas, and in some cases, we have $10-per-pound material that is used infrequently clumping between days it’s used. We’re looking for ways to rescue clumped material and any other ideas to use, such as desiccants, when the powder isn’t in the air-conditioned storage. C.J., Markle, Ind.
For some things, there are no substitutes. However, if you have agglomerated material, you can run it through a sieve, be it handheld or powered, and remove a great deal of the lumps, but not all. In the long run, the cheapest way to avoid agglomerated powder (due to moisture) is to place the material is an environmentally controlled atmosphere. Maybe that’s the bosses’ office, or whatever. You’re going to spend a lot of time (and money) trying to eliminate it from the system. For the boxed powder, you should get some air conditioning. Desiccant absorbers don’t work very well in this situation. That’s because the preferred storage and usage humidity is in the 20 to 60 percent range, and desiccant canisters or bags aren’t that sophisticated. The most common way to condition the powder that’s in the hoppers is to leave the fluidization air on all the time, including weekends and overnight. This is predicated on your having good, dry compressed air. If the compressed air is damp, don’t bother because it’ll make the situation worse. Hey, suck it up, take the bosses’ credit card and go to Wal-Mart or H.H. Gregg to buy a room air conditioner and put it in the storage room. And make sure you size the AC for the room. —G.T.
Q: We’ve just started a powder coating operation. We had mostly sprayed powder on structured components. Recently, a customer required a plain polyester powder coating in star white. With this type of powder, however, we had pinholes on the surface. We tried some tactics to solve the problem. For example, we checked the dryer for the powder coating plant, and it works fine. We checked the spray application equipment, and cleaned the booth and the area thoroughly. We’re still getting pinholes. Please help us. P.B., Navi Mumbai, Mahape, India
You left out some information. What is the substrate? Is it a material that might be out-gassing? Or is the cleaning method leaving something on the surface that causes pinholes? And how old and under what conditions is the powder stored? It might be old or gone bad. This isn’t likely, but it’s something that should be eliminated from your search for an answer. I don’t know with any certainty what you mean by “structured” components. All things being proper, you shouldn’t have any problems applying polyester. Maybe you have an equipment contamination problem that just doesn’t show up to the naked eye when you examine the parts that are “structured.” —G.T.
Q: I need to coat copper tubing (refrigeration) for immersion in water (tube and drum water chiller). The coating needs to be durable, yet able to flex with the copper tubing. Any ideas? Thank you. B.A., Wrangell, Ark.
Years ago I coated some copper tube for use in a refrigeration unit. I used an epoxy material for that situation, partly because the epoxy was what I had on hand and partly because the epoxy was flexible enough to move with the tubing, yet retain its adhesion, and it was never going to see sunlight, so UV wasn’t an issue. Just in case bad things started to grow in the water, the epoxy was pretty resistant to whatever might take life. Okay, so now you’ve heard a tale from the past. It would still apply though: An epoxy material will hold up while immersed in water, and you won’t need any UV resistance either. Bear in mind that the copper must be very clean for good adhesion. —G.T.
Q: I apply a super-durable TGIC-based (triglycidyl isocyanurate) polyester powder on extruded aluminum parts. Currently, I claim that the coating meets American Architectural Manufacturers Association (AAMA) standard specification 2604-02. This states that a panel exposed to South Florida weathering will retain a minimum gloss of 30 percent and change a maximum of 5 delta E units over a 5-year period. Recently, I began using a new powder supplier, and I can’t wait 5 years for test results. I would like to develop a specification that uses accelerated weathering as the test method, but I’m not sure what performance objectives to claim or which accelerated test to use. Are standard specs already written for this type of weathering? If not, are there performance benchmarks I could use as a starting point (perhaps in the automotive industry)? A.G., Philadelphia, Pa.
Several papers have been written in regards to Florida weathering studies on super-durable TGIC-based polyesters. One of these papers compares and correlates between Florida weathering data and artificial weathering with equipment such as QUV A-340 and Weather-O-Meter. There are claims that a given number of QUV A-340 exposure hours are equivalent to 1 year of Florida weathering. It’s possible that 1,000 hours of Weather-O-Meter exposure with the 102/18 program, which is 102 minutes of ultraviolet (UV) light and 18 minutes of deionized (DI) water sprinkling, is equivalent to 1 year of Florida exposure. So, you could look at somewhere around 200 days of artificial weathering test duration, which is about 7 months. This is a long time to wait on results. You should compare the high outdoor durability of the powder from the previous supplier side by side with the powder from the new supplier. We discourage the use of the faster and higher intensity artificial weathering test procedure (QUV B-313 ). It has been proved that these tests don’t correlate well with actual outdoor weathering. The automotive industry is replacing these tests with artificial weathering testing as mentioned above. Ask the new supplier to guarantee the powder against the AAMA 2604-02 weathering specification. If the supplier doesn’t want to do that, then you better go back to what has been proved to work. You know the old story: You get what you pay for! —G.T.
Q: I have some automotive car parts (sheet metal fog lights) in need of powder coating. I’ve bead-blasted the housings, smoothed the surfaces with 380-grit sanding disks, and blended and smoothed with a Scotch-Brite abrasive wheel. Is it possible to fill the remaining rust pits with a metal-filled epoxy? How will the electrostatic attraction be affected, or do you have suggestions? B.P., Tampa, Fla.
The metal-filled epoxy should attract the powder. However, in a finite diagnosis, if the metal fillers don’t touch one another and they don’t ever touch the base metal, it’s conceivable that the coating would be light. You could consider pre-heating the part to about 110°F; however, it will cause some heavier film build on the remainder of the part. Actually, I think you won’t have any problems with what you’re doing. The filler will be pretty thin, so coverage should be uniform. —G.T.
Q: We’re currently using epoxy powder coating and testing for sufficient cure by using methyl ethyl ketone (MEK). We have an initiative to get MEK out of the facility for environmental health and safety reasons. We have received recommendations of toluene and xylene, which aren’t acceptable either. Do you know of any chemical that will be aggressive enough to verify cure without being a total toxic organic or a carcinogen? C.B., Reading, Pa.
Plain and simple: No, there’s no substitute for a reasonably quick check on cure. If you follow the Powder Coating Institute’s “#8 Recommended Procedure for Solvent Cure Test,” you’ll use very little MEK, and it can be stored safely in a quart can in a safety cabinet. Several other tests are available, although each has its drawback. • The glacial acetic acid test uses 96 percent, or higher, concentrated acetic acid. It’s applied in one drop on the film surface. After 60 seconds, the surface is wiped off, and the film is judged for loss of gloss, softening, discoloration, and so forth. It’s very similar to the standard MEK test, but glacial acetic acid is very corrosive, and it can cause exposed skin to blister after a short exposure. It seems to be worse than the MEK test. • Another test that is destructive and not very accurate is the scrape test. In this case, you would cut into the paint film down to the substrate and then drag the blade along the film for about an inch. Undercured films will break away easily from the substrate. It takes an experienced person to establish a value to this test, and even then, it’s a bit subjective. • The most accurate test, one which will hold up in a court of law should it ever come to that, is a differential scanning calorimetry test. In this case, 5 to 10 milligrams of scrapings are removed from the film and tested against a known uncured sample of the same powder material. The results will actually give you the percent of cure by comparing the exothermic reaction of the undercured material with the suspected film. The standard coating performance tests for hardness, impact resistance, color fading, and so on will also give you a clue as to whether the coating is cured. Of course, all of this takes time. If I were you, I’d give all of this information to the department that has insisted on the removal of a very, very small quantity of MEK, and tell them that they can bear the burden of the budget that will give you an alternate method of testing parts for cure. —G.T.
Q: We’re looking for a powder that meets Mil d 24712. Do you know any suppliers who do the testing on their powder to meet this specification? M.B., Holliston, Mass.
I think the spec you’re referring to is Mil-C-24712, not Mil d 24712. This is the generic powder coating specification developed by the US government to describe most all common powder coatings. Most powder coatings should meet this spec, but most powder coating suppliers don’t bother trying to “qualify” their material to this spec. It covers the following generic powder formulations: epoxy, epoxy-polyester hybrid, polyester, triglycidyl isocyanurate (TGIC) polyester, polyester urethane hybrid, acrylic, acrylic-polyester hybrid, acrylic urethane hybrid, urethane, polyurethane, vinyl, and nylon. That about covers the entire gamut of materials in our industry other than polyvinylidene fluoride (PVDF) and Teflon.
Go to this publication’s Web site www.pcoating.com and click on Online Buyers Guide at the left. Search under Powder Coatings for military spec powder coatings to contact a supplier that probably has materials to meet this very generous spec. —N.L.
Q: I’ve been having some trouble with clear powder coating over a veined powder coating. After the part leaves the shop, customers complain about the clear cracking or spider-webbing. I’ve talked with the powder supplier, and I was told that this is a common occurrence. Could you also discuss salt-spray testing. I would appreciate any help with this. B.C., Grants Pass, Ore.
Most often, this condition is caused by surface tension because of incompatibility between the clear and the basecoat. The two must get along with one another, and your powder supplier should be able to correct this for you. If your supplier didn’t mention it, maybe you’re using different suppliers for the two coatings. What is it you want to know about salt-spray testing? The longer the hours in the test without failure, the longer the service life of the coating as far as adhesion is concerned. You can test panels with a scribe or without a scribe. The most severe test is an X scribe across the panel. You then check for creep into the coating from the scribe mark. The creep allowance may be no creep, 1/8-inch creep, or ¼-inch creep. The more liberal the specification, the shorter the adhesion time of the coating. This isn’t a test for abrasion, dents, gloss resistance, or some other physical characteristics of powder coatings. —G.T.
Q: Q1: I’d like to powder coat the frame of an experimental aircraft, but I’ve heard that the powder coating process can weaken the metal. The metal involved is 6061 T-6 aluminum. I’ve also read that the temperature and cure times can be set to avoid any change in the alloy. What are the correct parameters for maintaining the structural integrity of this aluminum alloy? R.M., Bradenton, Fla. Q2: Do you have any information, recommendation, or advice regarding the concerns that powder coating and the temperatures used will weaken or affect the strength of forged rims. T.T., Kapolei, Hawaii
I’ll answer both R.M. and T. here because their questions are related. Organic thermoset powder coatings (what most people know as standard decorative powder coatings) have cure temperatures between 250°F and 450°F. The cure times at this temperature vary but can be as much as 30 minutes for the lower temperatures and as fast as 10 minutes for the higher temperatures. Pretty much all ferrous materials (steel and iron–tempered or not) have no difficulty with these temperatures and times. However, tempered aluminum (6061 T-6 is tempered aluminum) and softer metals (copper, lead, tin, etc.) can have problems with temperatures above 350°F. The metal temper can be annealed (softened), or the metal may begin melting if it’s subjected to temperatures higher than 350°F. Therefore, any products that have these materials must be cured at temperatures below 350°F, even if it takes longer to complete the cure of the particular powder coating.
@b1:T., you have no worries with your forged steel. However, R.M., you shouldn’t have your parts cured with powder coating above 325°F just to be sure you don’t soften your aircraft structure. You should also be aware that powder coatings are heavier than liquid coatings because they’re applied much thicker. This fact may also cause some issues with adding unnecessary weight to your aircraft as well. —N.L.
Q: When I apply a second coat of powder onto a part, the powder doesn’t cover the side of it. I have an inexpensive hobby-type gun, and I think the voltage isn’t enough for make a good job on the second coat. Is this possible or not? C.T., Victoriaville, Que.
The equipment you’re using isn’t intended for anything more than small hobbyist applications. If you need to get a bit more sophisticated in your coating results, then you should look into an industrial application unit that’s capable of doing everything you want. You can find this equipment used sometimes, but it will take you some searching to find one. They are available. —G.T.
Q: We’ve been custom coaters in Phoenix since the 1980s. We’ve shot powders from the RAL palette since the beginning. We’ve seen improvements in powder quality across the board from all major powder manufacturers, in all formulations. But what has not improved is the uniformity in matching RAL colors from supplier to supplier. They’re all kind of close, but all do not match. Is there any effort by powder manufacturers who sell the RAL palette to come up with standardized RAL colors (similar to Federal Standard) so they all match from supplier to supplier? R.Q., Phoenix, Ariz.
RAL color standards can be very useful in obtaining the same color from different suppliers. However, you must also specify the tolerance you allow the supplier to drift from these standards. For this, we normally use a Cie-LAB standard of 1 ΔE, which is undetectable to the naked eye under most lighting conditions. Some colors may also require a maximum value for each of the L, A, & B values in addition to the ΔE value to ensure color accuracy.
Work with your suppliers to establish your own color standards and tolerances to ensure you obtain consistent results. —N.L.
Q: I’m using sandblasting media, air blowing with compressed air to clean sand out, then cleaning with a solvent, like a paint thinner or alcohol. I then heat the part dry for 15-30 minutes at 370°F (to outgas and dry the part), and then I remove the part from the oven and let it cool to around 150°F or sometimes cooler (tried different temps). The aluminum is usually cast, mostly motorcycle parts. I’ve been told that the pinholes were caused from out-gassing so that’s why I started doing the heating before powder application. It seemed to help, but it hasn’t eliminated the problem. C.S., Windsor, Ont.
Castings, be they ferrous or non-ferrous, will usually cause out-gassing because the metal is porous, as you know. And if the metal has been subjected to a chemical pretreatment, it’s entirely possible that some of the liquid has entered and become trapped in the pores. When heat gets to the part, as in a cure oven, the air will expand and seek a way out, and the liquid trappings can approach boiling and erupt through the coating. Pre-heating may or may not solve this problem. It will certainly reduce the amount of bubbles in the coating. Some years back, I worked with a research type engineer, and his company had a lot of problems with out-gassing. He determined that time was as important as temperature in the preheat stage. Given enough time, you can virtually eliminate the problem. However, in a production mode, you don’t have the time to remove all of the gasses/liquids. So, a compromise is necessary. Sometimes, coating thickness will inhibit the out-gassing, but that adds to cost and can affect appearance. A primer will serve the same purpose and could be the best means of solving the problem. —G.T.
Q: I have a customer who wants us to put THREE coats of our TGIC powder paint onto his sheet metal parts. That seems like a disaster waiting to happen. We cure the paint at around 350°F (the max our oven will go to). Do three coats sound reasonable? What is the maximum paint thickness recommended with these powder paints? J.B., Tulsa, Okla.
Q: Just wondering if you can help with a problem we’re having with our powder not attracting to our parts. As far as we can tell, we’re getting a good ground on our parts that are hanging in our booth, but when we spray the powder on the parts, you can see powder just kind of falling off. And if we try to spray in corners or anywhere there is a bend, the powder won’t stick. This is causing us to overspray the parts to get full coverage, and the powder is very thick after the parts are baked. Our operator for our powder coating said it looks like the powder wants to attract directly to the booth instead of the parts. We’ve tried adjusting our settings on the powder coating machine, but nothing seems to help. Your help would be appreciated. C.M., Lewiston, N.Y.
From here, this is a classic case of little or no ground. It bothers me that you say “as far as you can tell, we have good ground.” You should know precisely whether you have good ground or not. You need a megger meter, or megohm meter, to easily take a reading at the various points along the part stack. If you have 1.0 megohm or less, you have a good ground. Anything over that will gradually be less attractive to the powder. Another way to check ground is to make up a ground wire with a set of alligator clips on each end, and connect to the part and a KNOWN ground. You should see a marked difference. If that isn’t the answer, then the problem immediately gets a bit weird. It could be a powder problem. If the powder is old, improperly stored, or damp (you should see other problems if this is the case), or had a problem right from the start, then there are other ways to test for the cause. However, it sure sounds like a ground problem to me. —G.T.
Q: Are there cool powders? We powder coat aluminum extrusions for making doors and windows, and we want a powder that will reflect the infrared radiation, and the window will remain cool. Let me know how they work and where I can buy them. L.K., Piraeus, Attica, Greece
All of us in the finishing industry think powder coatings are way cool! The physics behind your question are pretty simple: Dark colors absorb light; light colors reflect light. Light absorption will result in heat energy; therefore, architects for years have selected lighter colors for their buildings in climates where light reflection aids in reducing building energy requirements. Selecting metallic colors can further improve light reflectance, which is why most energy-efficient buildings have highly reflective colors on their exterior surface.
In your application, the best solution is not to apply any color at all but use a clear powder to allow the underlying aluminum color to reflect the sunlight. —N.L.
Q: We’re coating automotive components and are in the process of trying to complete a new order for coating shock absorber springs for motorcycles. The powder is repelling from the inner side of the spring in some areas and pin-holing. Because we try to reach the inner side to coat it, we get excessive coating on the outer portion, which isn’t acceptable to our client. What should I do to resolve the coating problem for the inner side of the spring? S.D., Aurangabad, Maharashtra, India
The pinholes could be caused by improper cleaning of the inner portion of the spring. Some oils may still be present. Or, if the space is really tight, you may be getting kilovolt rejection because the gun/electrode is too close to the metal. Springs should be easy to spray. I haven’t tried bike springs, but I have sprayed auto coil springs. You can use one gun to spray the entire spring without moving the spring or the gun. Anyway, spray the interior first, then lightly spray the outside. You can take a good look at the springs when you coat only the interior to get a good idea how much powder will be needed to finish the exterior. I would think that it would take only a light pass to finish coating. If I’m not getting the picture, then take a few photos and e-mail them to me so that I can take another stab at it. —G.T.
Q: I was wondering if you can paint over a powder-coated surface. If so, which type of paint is better–enamel or acrylic? Thanks in advance. S.J., North Ryde, New South Wales, Australia
It’s an industry-accepted standard to touch up powder coating with a good quality touch-up paint. It’s been that way for years. Try very hard not to sand defects to bare metal. Just scuff it. If you sand to bare metal, you should apply a pretreatment chemical to the bare metal before painting. These treatment chemicals may be purchased at most automotive refinishing stores. Enamel will work just fine; however, if your product is used in a corrosive atmosphere, the touch-up should be a two-component material. This of course makes the touch-up a bit more complicated and cumbersome, but it’s the best practice. —G.T.
Q: What percentage (maximum) of recovery powder can be mixed to virgin powder with the same characteristics or same appearance after baking? What are the advantages and disadvantages of recoating parts such as rejects and how many coatings could be applied? V.M., Quezon City, Rizal, Philippines
The industry standard for application efficiency is 60 percent. This means that about 40 percent of the powder will go to reclaim. Part configuration and hanger design will impact this efficiency either up or down. Anyway, 40 percent reclaim will normally not cause a problem. If you get more than 50 percent reclaim, you’ll have fluidization problems and impact problems in the hoses and guns, and you’ll have a modified appearance on the parts. Powder chemistry and age can also have affect the reclaim, which will in-turn affect the appearance of the powder on the parts. The number of recoats allowed will depend to a certain degree on the use of the parts. If there are any machined areas, or fit areas with other parts, you shouldn’t apply more than a second coat, depending on the film thickness of the first coat. Orange peel will become more prominent with additional film; therefore, if appearance standards are strict, you may not wish to do a recoat. If the reject is small, you can do a liquid touch-up with a color-matched material, but you’ll have to be the judge of that. —G.T.
Q: We are a small effect powder coater and thermal barrier (ceramics, etc.) applier in Germany. We also sell powder coating machines as a niche product solution for small shops. We are always trying to get a perfect solution for sealing our coatings. We are now at a pretty good stage where we use an industrial one-component nano clear seal (wet paint) that is also cured at 180°C (356°F) for half an hour. Do USA coaters have experience with coatings like that? Can you tell me which powder producers supply powder coatings that produce a real chrome look that can be sealed without fading into yellow or gray? M.W., Luebeck, Germany
First, I’m not sure what you mean by nano. If you mean an organic clear coat, then, yes, we have that here in both powder and liquid form. Powder clear is used on automobile wheels, plumbing fixtures, and many other items. Liquid clear is used on automobiles, bikes, and many other items as well. I’m reasonably certain that nano isn’t a ceramic material because the temperatures aren’t high enough. To find a powder supplier for chrome lookalike powders, you should try the list of powder coatings manufacturers in the magazine’s online Buyer’s Resource. —G.T.
Q: We currently run three lines with automatic spray guns and a reclaim system. My question concerns particle size. When does the particle size become a safety concern? What’s the best method of rotating recycled powder so that it doesn’t become a safety hazard or affect the chargeability/transfer efficiency? I thank you for your assistance. J.A., Kowloon, Hong Kong
This is a bit like powder coating 101. The overspray is made up of mostly smaller particles because they don’t get a charge like the larger particles do. So that’s the dominant particle size of the reclaim. I hope your powder system efficiency is at least 60 percent. Much less than that and you’ll have material handling problems. You shouldn’t have any issues with smaller particles if you’re using a 60 percent virgin to 40 percent reclaim ratio. Too many small particles will affect the cured powder appearance some, not a lot, but some. More important, too much reclaim, that is, small particles, will cause fluidization and application problems. It can get so bad that you won’t be able to fluidize the hopper or move the powder to the guns. The small particles in themselves won’t be a safety problem. Although I’m not sure what your idea of a safety problem is, they will have some small affect on dust-to-air ratios. When this happens, you can risk an explosion or fire, but this will occur when a spark is present. So, if you have a dirty hanger, and the ground is poor, there will be an arc between the gun electrode and the hanger/part. This can cause a fire. An explosion is extremely rare, and a fire is unlikely if you have the proper spark detection systems installed. Now hear this: The particles don’t have to be small for this to occur. It can and usually is regular powder particles that are involved. And if the hangers and conveyor are grounded properly, you won’t have a problem. —G.T.
Q: I have just had my engine and exhaust powder coated. The engine is okay so far, but the down pipes have bubbled up. Why is this? Many thanks. D.D., Cirencester, England
Because the powder coater did not use a heat-resistant powder coating on the exhaust. Eventually, all the powder coating on your exhaust will flake off the parts. Next time, request a dual resin with silicone heat-resistant powder coating formulated for exhaust systems (and gas grilles for that matter). Color choice will be limited, but at least it won’t flake off when the coating is used on parts where the heat is above 275°F. —N.L.
Q: I’m interested in getting feedback on coating plastic. A lot of people are looking at coating plastic based on heating the plastic first. I think there can be irregularities not only in the film build, but also in the gloss and surface of the powder, smooth or textured. Just wanted to let you know, you give great information. C.J., Markle, Ind.
Coating of plastics is a challenge, and I find it interesting that some people seem to think they have an answer and others don’t. Sometimes, just a minuscule thing makes the difference. When I was working on an experiment for a plastic car body, we tried a liquid spray-on material that was purported to enhance electrostatics. If it did, we didn’t notice it, and preheating of the parts became a necessity. We worked many long hours trying to arrive at a proper preheat temperature. Part of the problem was, immediately after preheat, we had to get the body into the powder booth. Well, the powder booth needed to have the reclaim blower on, so it was introducing ambient air under force, which began to cool the part. Allowing for this made for a lot of trial and error on the correct preheat temperature. We sprayed powder with the electrostatics on because the fan pattern was enhanced by the electrostatic field. This effort helped in uniform distribution of the powder. The powder film was within 0.03 mil from side to side on the car body. There are companies that are applying powder coating to plastic bottles. The bottles are on a spindle and rotate, so application is relatively simple. The bottles are preheated in some cases. In other cases, they’re sprayed with the aforementioned electrostatic enhancer. Each of the companies thinks its way is best. Well, who knows. Gloss and powder surface will be affected by the thickness of the plastic because of reinforcing or just plain variances in the plastic. This will show up directly in the powder, usually where preheating is the attraction for the powder. —G.T.
Q: I have some automotive headers and would like them to be powder-coated in flat white that will sustain header heat. I know of ceramic coatings, but that is another process. Any suggestions? B.P., St. Paul, Minn.
High-temperature organic materials are available. Even though these materials are designed for high temps, they’re good for only so long before they will begin to burn. That’s one of the reasons the serious show car exhibitors trailer their vehicles. The pipes never get to sustained high temps. —G.T.
Q: We cut, drill, and buff extruded aluminum parts in a variety of shapes in-house and then powder coat them. We’ve had a higher number of rejects than usual lately because of aluminum chips on the parts. We paint a lot of metallic and mica powder. We installed bag filters and 250-micron filters in screens, but we still see aluminum chips on the parts (under and over the paint). We noticed aluminum flakes and other airborne dirt floating in the cure oven. The aluminum chips show up only on aluminum parts and not on steel parts. What do you recommend we do to determine the cause? Is it coming from the wash, the spray-gun tip, or oven airflow or vibration? K.M., Holland Landing, Ont.
Contaminants in powder-coated products can come from numerous sources. Looking at your coated and cured products will help identify what the sources of these contaminants are. For instance, contaminants on the surface of the coating (not in or under the coating) come from sources after the coating operation (applying the powder). Conversely, contaminants in or under the coating can come from sources either before powder application or during powder application.
Contaminants on top of the coating are typically deposited in the cure oven and are airborne in nature. Contaminants in or under the coating can be mixed in with the reclaimed powder and applied before or during powder application. Visual inspection of parts before they enter the powder coating booth will tell you if there are contaminants on the parts before spray. Finally, spraying clean test panels with reclaimed powder will tell you if your reclaim is contaminated.
Once you’ve identified where the contaminants are in your coating process, you can begin to eliminate them as coating defects. The best way to eliminate these contaminants is to trap and collect them at their source, which in your case in at the fabrication stage. Air hoods and dust containment systems will contain and collect contaminants as they are made by machining operations.
If you can’t eliminate the contaminants at their source, then isolate your powder coating process from your manufacturing process. This is done by installing an environmentally controlled room around the powder coating process. This room should be made of easy-to-clean materials and have positive air pressure to the surrounding plant airspace to keep airborne contaminants from entering the room.
Finally, it’s unlikely that these contaminants are being deposited by your wash system. However, your wash system may not be removing them entirely from the part surface. Increasing dwell time and impingement pressure should fix this issue. —N.L.
Q: Where can I get a powder production process manual? What is the most recommended warehouse storage temperature? J.E., Apodaca, Nuevo Leon, Mexico
Several universities offer symposiums on the formulation of powder coatings and the operation of powder processing equipment. However, I don’t know of any handbooks on the subject. There is a white paper available from Bruno Fawer, associate consultant at Powder Coating Consultants, that may shed some light on your questions. You can reach him by emailing email@example.com.
As for the best storage temperature for a warehouse containing powder coatings, I recommend 70°F to 80°F for most normal formulations. Higher reactive systems (low-temperature-cure powders) should be stored at 60°F to 70°F. —N.L.
Q: What’s the best way to get rid of used powder when we spray to waste? Is there anyone that uses this powder? Right now we put it in the dry-off oven until it gets hard, then throw it away. J., Peoria Heights, Ill.
That’s the best way unless you can find someone who’ll buy it from you. Finding these people isn’t easy because it usually requires some type of sifting or filtering system to clean up the material. If you have a lot, maybe you should consider a reclaim system for high-use powders. —G.T.
Q: What’s the most effective way to remove old powder coating without having to invest in chemical process equipment? I’ve tried media blasting with coal slag, chemicals called “stripper” in an aerosol can, and aircraft paint remover in a liquid form. Most of the products I powder coat are new steel or aluminum, which I prep by media blasting. This is the first time I’ve had to deal with already powder-coated parts. It’s a pain! Are there different removers for steel and aluminum? D.K., Bagley, Minn.
There aren’t different strippers for aluminum and steel that I know of at least. A myriad of companies claim to have metal strippers. Some work, some don’t. The two most effective ways to clean cured powder from a part is by burn-off (not good for aluminum, depending upon the parts) and chemical. Stripper in a can absolutely won’t work as you have apparently found out. The hot-sand method is a variant on the burn-off method. Parts are placed in a container with sand in it and a burner tube. The sand fluidizes, and the heat aids and abets the sand in removing the coating. You’re finding out why, when properly applied, powder is such a good coating. Depending upon the number of parts you have to strip, search for a stripping company in the area and have the company chemically remove the coating. Once the powder is removed from the aluminum, make sure the parts are cleaned of any residuals. Then, pretreat and paint immediately. Aluminum will begin to create its own patina very quickly, and this will negatively affect the adhesion of the paint you’ll apply. —G.T.
Q: I have a part that has two issues. First, the part is stainless steel. Second, it requires 10 to 15 mils of paint. What issues can I expect when powder coating stainless steel, and how difficult will it be to achieve the thickness spec? B.S., Toronto, Ont.
Stainless steel comes in several finishes. The most common is a high-gloss surface. Less common is a muted finish. I don’t suppose you’re lucky enough to be coating the lower-gloss material, which is friendlier to paint than the high-gloss finish. For a lasting finish, you should scuff-sand the area to be coated; otherwise, the finish will scratch easily, and the paint will begin to peel. Depending on the designed film thickness of the powder you’re using, you’ll probably have to build the film in several passes. Trying to get the coating that thick in one pass will most likely create “starring,” or kilovolt rejection. You can apply about 4 mils, maybe 5, in one pass. Place the part in the oven and set the powder, then coat again. Continue until you get the desired film thickness. It certainly would help if the powder is designed for heavy film build. If that’s the case, then you may be able to achieve the thickness in no more than two passes. If you’re going to use an off-the-shelf material, it may require multiple passes. This won’t be a cheap application. I hope that you’ve made your customer aware of the cost. —G.T.
Q: What temperature should galvanized steel reach when degassing it before powder coating over it? S.M., Louisville, Ky.
Preheating galvanized steel may or may not completely degas the substrate. You should experiment with time and temperature settings to eliminate most of the gas. A good starting point is to select the cure temperature and cure time for the powder coating you’re using first and go from there.
Beyond the gas problem, make sure the surface has been “brush blasted” or chemically etched to remove the zinc oxide on the galvanized surface before applying the powder coating. Otherwise, you’ll have an adhesion problem with the powder coating. —N.L.
Q: We’re looking for a material that could be machined and used as part of our powder racks for masking areas of product where no powder is allowed. Masking a part is too costly. We would like this protection to be part of the racks. We have the ability to machine and build special racks. We have a standard powder process with a five-stage washer. We cure parts up to 475°F in our cure oven. B.J., Jasper, Ind.
I know of no material that will resist powder coating and serve as a mask at the same time. The problem is that some things you could use, such as petroleum jelly (shudder), will cause horrendous problems in a powder coating shop. Will petroleum jelly keep the powder off the area? Sure, but then it has to be cleaned off, to say nothing of the contamination problems it would cause. Plastics, such as nylon and a dozen other synthetics will protect the area and survive the heat, for a while, but they have to be cleaned or tossed out and replaced. Exotic metals? They would still draw the electrostatics, and they’re too expensive. So, here are several suggestions. You probably won’t like some of them. You can powder coat first and then machine. It’s done all the time (on engine blocks for example). You can vacuum away the powder from the critical area. This works nicely when the person doing it cares, or if you’re lucky, you can automate. Maybe the critical area can be shielded by something on the hanger. Bad news here is if it needs to be a tight fit, it probably can’t be done. And the hanger will need frequent cleaning. Don’t even think of making a moving piece as part of the hanger because that will get powder coated and cease to function. I suspect you’ll just have to bite the bullet and try some version of the above mentioned items. —G.T.
Q: After powder coating, especially with black color, the powder coating becomes white after a period of time. The parts are mild steel. M.A., Mafraq, Jordan
The problem you describe is called chalking. Powder coating formulas that aren’t formulated for outdoor exposure (ultraviolet-light [UV] resistance) will readily turn from black to white in a relatively short time (a couple of weeks). Epoxies and epoxy-polyester hybrids aren’t formulated for UV exposure and will readily chalk in sunlight.
Select a coating formula that has the UV resistance you need, that is, a polyester or an acrylic formula. These formulas are manufactured to provide excellent UV resistance, meaning that both the resin and the pigments are UV-stabilized. —N.L.
Q: I was wondering if you have any idea where we could have independent testing done on some aluminum parts. We have reports of the paint peeling off aluminum rails and need to find out why this is occurring. The powder we’re using is a custom color through our regular powder distributor, which is the only color we’re having trouble with. I’m hesitant to have the distributor test the powder because I’m not sure how impartial it would be. I thought of our chemical company, but if it’s a problem on that end, I may not receive accurate information either. Any information you can provide will be appreciated. B.F., Hamilton Township, N.J.
The most common reason for a coating to peel off of a substrate is poor metal pretreatment. Actually, that’s about 95 percent of the reason. If you have failure on aluminum, that percentage rises to about 99.9 percent. You have to clean aluminum before coating it. You don’t say what process you’re using, if any, but a description of your current system would be enlightening. There are several sources for testing parts or panels. One is the paint guy. Another is the pretreatment guy. And another is an independent laboratory. The latter is very expensive, which makes one or both of the previous two attractive. I could find a lab for you, but be prepared to pay in the four-figure range for the lab services. If you’ll send a description of your current metal prep method, maybe I can at least eliminate some things that could be at the root of the problem. —G.T.
Q: We’ve had to put on some 2-inch pieces along the top rail of a trailer. When we powder coat it, we seem to blow the powder off the inside instead of making it stick. We’ve tried to move the gun farther away, and we lowered the airflow. The ground seems to be okay while the rest of the trailer is coated. I’m sort of at a loss here. It’s just a very tight space, and there isn’t a way for us to coat it unless we get the gun down inside of that space. A.B., Hugoton, Kans.
Could it be that the main trailer body has a good ground, but when attaching the rail, you lose or greatly reduce the ground? You might try running a ground strap directly to the railing. Beyond that it just might be that this isn’t a good application for powder given the setup. Can you coat the railing before attaching it to the trailer and then touch up the attachment point? —G.T.
Q: Our company specializes in fabrication of aluminum doors and windows. Seven years ago we opened a powder coating plant. Now our finished product produces a ring, or crater, throughout the surface of the aluminum. My question is how to solve this problem. A.N., Dammam, Kingdom of Saudi Arabia
Craters are typically caused by an organic contaminant on the part surface, in the compressed air, or in the surrounding environment. Eliminate this contaminant, and the craters will disappear. Look at your cleaning system to remove all the contaminants on the part surface and verify that you have a water-break-free surface. Ensure that you have a good oil separator on your compressed-air system. Your compressed air must be oil- and moisture- free (lower than a 32°F dew point). Finally, isolate your powder coating operation from surrounding manufacturing operations that may be contaminating the air in, or around, the powder coating process. —N.L.
Q: Some of our customers ask for a color change over already powder-coated parts. We have noticed that some parts accept the deposit; others don’t. Why? Is there anything you can recommend to activate the powder coating before recoating with another coat of powder? A.M., Caguas, Puerto Rico
There are several things you didn’t tell me about your rework process. Are you scuff-sanding the original powder coating before applying the new color? Are the new colors in a powder formula that is compatible with the original? If there is a surface-tension condition between the two powders, you might never be able to get a good second recoat. Make sure you do a good sanding job. Then, run the parts back through the washer with the phosphate off, if there are no bare metal spots, and then recoat. Be alert for any water spots that will leave “salts” on the surface and cause a reject. If you don’t have a washer, solvent wipe the part with a clean cloth after scuffing, allow it to dry, and then recoat it. If this doesn’t do it, then you likely can’t avoid the problem. All recoat powders should be from the same supplier preferably, and they must be of the same resin family. —G.T.
Q: We’re having some issues with parts that have to be spot-welded. The process creates a mark on the metal. This dragging mark bleeds through the powder coating only on light, smooth colors. We had this issue occur 2 or 3 years ago, and I blamed it on the spot-weld process. Now, I’m not sure. I hope you can give me an idea of what’s causing this. Thank you very much. N.M., Brea, Calif.
Not all powder coatings have the same hiding power characteristics. Generally, smoother, higher gloss coatings will highlight metal surface defects like scratches, spot welds, and so on. Conversely, lower gloss and textured powder coatings are much better at hiding surface defects. You have three choices. One is to improve your manufacturing process to eliminate surface defects on the metal before painting. Another way is to change your coating to better hide the defect. If improving the manufacturing process at the source isn’t practical, then adding another step to sand the surface smooth before powder coating is another choice. Selecting a powder coating with better hiding power can be as simple as changing the gloss or increasing the orange peel (up to and including textures, wrinkles, and so on) to hide the defects. —N.L.
Q: What happens when you spray a powder marked tribo with a corona-charging system? I’ve special-ordered a powder from the US for a very important and rush job. The label says tribo. Just wondering if I can still spray the powder with my corona-charging system. B.D., Calgary, Alta.
You shouldn’t have any unusual problems spraying this material with a corona-charging system. The label is marked that way because there is likely a problem spraying just any old powder in a tribo-charging system. Powders for use in a tribo line are formulated for that purpose. There has to be additives that will enhance the natural static charge of the powder passing through a tribo-charging gun. This isn’t a requirement for corona-charging guns. So, you can use it; you’re just wasting the modified formula normally reserved for tribo. —G.T.
Q: We’re processing cold rolled steel (CRS) panels that have been spot-welded with cosmetic spot welding tips. We prep the panels by grinding away the raised spot welds and then by finishing with 120-grit paper. We’re using a white urethane-based polyester powder coating, and our base metal temperature is 400°F for 10 minutes for full cure. The problem is we’re seeing spot welds after the coating is applied. Is there a filler that works for powders? We’ve tried some in the past with very limited success. D.W., Salt Lake City, Utah
Powder, like any other paint, won’t fill in imperfections in a surface. In fact, it will tend to highlight them. Many times if there is a “crack” around the defect, the powder will flow away from the crack and really accentuate it. There are body fillers of various substances that can be used for filling in voids, but you must make sure the filler can tolerate the bake temperature of the powder. Limited success is what you’re going to find with any of the fillers. I’m assuming that you’ve tried feathering the sanding in a broad area around the spot weld. If you haven’t tried it, give it a shot. —G.T.
Q: I have a problem. Some of the powder overspray has gotten on car upholstery, and I would like to know of a way to remove it. The temperature in the car was high but not enough to set the powder completely. We’ve tried many different things and nothing has worked. We need your help ASAP. R.D., Salt Lake City, Utah
Uncured powder coatings are very susceptible to solvent. Clean your car interior with a solvent that won’t hurt the fabrics or hard surfaces. Test the solvent in an area that isn’t easily seen to ensure that it won’t hurt the surface you want to clean. —N.L.
Q: We currently apply wet coating systems for miscellaneous equipment in industrial markets and have batch oven capabilities for curing phenolics, epoxies, vinyl esters, and fluoropolymer-type coating systems. We’re considering getting into custom (large, heavy, non-line) powder coating systems, such as zincs, epoxies, and TGIC-based polyesters, and would like to know what the downfalls of using some of our existing processes and equipment would be. Currently, we blast-clean items to a minimum of SP10 or NACE 2. We don’t have a pretreatment system other than solvent cleaning and blasting, and our original paint booth wasn’t designed for powder applications. Based on some of the responses I’ve read before in your columns, I can just imagine what’s going through your head. Can this be done, or should we look at having two different setups for applying wet and powder materials? Looking forward to your reply. T.M., Ontario, Canada
You can probably guess what at least some of my response will be. I’ll try to be gentle. Solvent wiping is like doing nothing, or the next thing to it. Blasting really, in theory, is merely cleaning the surface. You are doing no metal prep, and that is where you get rid of deep oils and condition the part surface for much improved adhesion and rust prevention. Blasting will not always remove some mill oils—not well enough for a water-break-free surface. And you won’t know about water-break conditions because you aren’t using a washer. With no washer, there are customers out there who will not or should not use your services. I’m not saying that’s bad. There may be enough business requiring a lower level of performance to keep you busy. That’s a business decision you need to weigh versus the cost of purchasing and operating a chemical pretreatment line. And there are environmental obstructions, too, depending upon the local regulations. This washer line can be used to service both the liquid and the powder business, and it would raise you a level or two for additional work. Okay, so much for pretreatment and other types of metal prep. You absolutely don’t want to mix powder and liquid in the same booth. You’ll be suicidal in a matter of minutes. You can use a similar type booth, one that doesn’t reclaim, but sprays to waste. You would need a dry-filter booth with a very good blower. You must contain the powder. It’s the law, and you don’t want to pay for repainting a lot of cars out in the parking lot. You consider how much spraying you’re going to do, and if it’s cost-effective to have or not to have a reclaim system. Since you plan to spray so many different resin base materials, it would be intelligent not to reclaim. Then, you just have to price your work to allow for spray-to-waste. You wouldn’t be the first person to do what you’re proposing. If you have good planning and keep your head clear and your pencil sharp, you can make it work. It’s not what I like, but hey, it’s not my money or my facility. —G.T.
Q: I’ve heard that powder booth cartridge filters can be cleaned and reconditioned. Is this true? If so, how good a job will they do after cleaning? Do you know of anyone who does this? N.N., Statesville, N.C.
You can remove the filter from the collector and place it on its side on a piece of throwaway material, such as paper or vinyl or whatever. Grab each end of the filter with a hand, lift the filter about 15 inches above the floor, and drop it. Rotate the filter about 6 inches and repeat this process. This will buy you some time, but the filter won’t be “like new.” This method doesn’t usually eliminate the powder that is deep in the recesses of the filter. You could place the filter in the booth and use compressed air to complete the job; however, the time spent doing all of this would probably buy you a new filter, depending on your hourly rate. Several companies in the US and Canada reclaim filters. I’ve seen them, and the filters look good. How well they hold up, I have no idea. Check the magazine’s Online Buyers Guide. —G.T.
Q: I’m having a problem with the powder coating looking like it has dirt in it. You can rub your hands across the surface and feel the imperfections. I’m preheating all my items before applying the powder. Any help would be great. R.G., Mooresville, N.C.
If it feels like dirt and looks like dirt, it probably is dirt. If the part is a casting, it could be outgassing bubbles, but with a hawk’s eye, you should be able to see the difference between dirt and outgassing. Pull a sample of powder from a sealed box of powder and spray it onto a cleaned piece of sheet metal to see if you get dirt. This exercise is to eliminate the possibility of a problem in the powder.
Can you see anything in the powder-coated part before curing? If you can, spray a clean, uncured part in the paint booth, or in a clean room, then send the part through the cure oven. You may also have air currents that are bringing in dirt that you don’t notice but is being attracted to the electrostatically charged part.
Now, if it’s outgassing, preheating is one way to reduce the amount of bubbles. You can also put on a heavy single coat. And you can spray a light coat, partially cure, and then finish spraying and completely cure the part to see if that doesn’t eliminate, or at least make tolerable, the amount of bubbles…if that’s what it really is! —G.T.
Q: I just had a custom lumber rack built for my truck, and the builder left out two small angle braces. Is there any way these pieces can be added, and the powder coating touched up without sandblasting the entire rack? T.S., Sebastopol, Calif.
You cannot field-repair your rack (after welding) with powder coating. However, you can use liquid coatings (spray paint) touch-up and end up with something that still looks good. However, this area will not perform to the same high quality as the surrounding powder-coated areas. This may not be a problem if the angle-brace fix occurs in an area of the rack that does not see severe abuse. If you want the entire rack to look and perform the same, then you will have to strip the rack, weld on the brackets, clean, and recoat it again. Sorry for the bad news. —N.L.
Q: I had a lower unit for an outboard motor powder coated recently. When I picked it up, it had what looked like small busted bubbles (pores) in the coating. The guy who did it said it was from the metal (aluminum) having pores in it. Is this guy BSing me because he did a poor job, or is it true? I wasn’t satisfied with his work, but he said it was unavoidable due to the metal condition. This condition isn’t over the whole piece but just in various places (some of them in places where I know the metal was smooth). E.W., Anacoco, La.
He wasn’t feeding you a line, at least not from where I sit. Aluminum will outgas because of the porosity of the metal. However, another chemical action is at work as well. Aluminum forms a self-protective skin much the same as a galvanized part will. For good powder adhesion and surface appearance, this skin must be removed before powder coating. If the aluminum is cleaned and then is allowed to set for much more than 4-6 hours, this skin will begin to reform, and it will interfere with a good finish. The way to finish any boat aluminum would be to chemically clean and chromate, apply a primer, and then apply a topcoat. The part should be allowed to dehydrate after the chromate application. Some heat may be used to accelerate the drying, but this heat must be limited to no more than 180°F. You probably won’t get all this refinishing work at a “local” shop, so your best bet is to sand (if required), apply a primer, and then apply a topcoat. The thickness of the powder will inhibit the bubbles from reaching the surface, and the finish will last much longer. —G.T.
Q: Has anyone power coated stainless steel air conditioning and power steering lines for automobiles? I want to have the stainless powder coated in a flat black finish. Any help will be appreciated. J.M., Poland, Ohio
If these are braided stainless steel lines, then forget about powder coating them because they will no longer be flexible after coating. Moreover, most small powder coating shops will have difficulty powder coating stainless steel because the powder does not easily bond to this surface. For these reasons, I would not recommend this idea. —N.L.
Q: We manufacture and powder coat threaded assemblies for our finished product. We’re having trouble controlling the powder thickness on these parts. We have explored caps and tape, and would prefer a different method. Is there a liquid or paste inhibitor we can apply to the threads to keep powder from sticking to them? J.B., Cerritos, Calif.
Your problems will only get worse if you use the materials you’re mentioning. Trust me. I’ve seen great success with the use of a vacuum to remove unwanted paint from threads and other machined areas. There are a variety of methods. Most often, you can affix a ¼-inch vinyl tube to a vacuum source and then, either by hand or by fixture, remove the powder from the critical areas. The vacuum source is a commercial type suction unit. Don’t use a Shop-Vac. The filter isn’t intended for particulate as fine as powder. Always, the end pickup unit is fabricated in your own shop. —G.T.
Q: I actually have a couple of questions. First, I recently had an issue with various parts all having paint flake off. I could take a razor blade and cut and actually pull flakes up. I’ve been unable to recreate this situation. Has anyone had this happen? I’ve had the powder, chemicals, and steel all tested. Nothing is standing out. Second, in an effort to cut costs and maximize throughput, we’ve been shutting our ovens down with parts in the line. Even though nothing is currently happening, are there any unforeseen issues by doing this? J.H., Pompano Beach, Fla.
The answer to your first question is “see question #2.” The answer to your second question is “see question #1.” Joking aside, if you have already verified that your pretreatment is good and your lack of adhesion comes and goes, then these situations can be linked. Powder coatings must wet-out on the metal substrate to provide good adhesion. This wet-out occurs early in the heat cycle in the cure oven when heat rapidly melts and flows the powder to wet-out on the substrate. If you stop parts in an oven that is shutdown, the heat energy dissipates quickly and the melting/flowing stops before proper wet-out occurs. Even if you start up your oven and preheat it to operating temperature before you start the conveyor, you may never attain proper wet-out of the powder on the surface.
In addition, interrupting the cure cycle by stopping parts in the oven during shutdown and start-up can lead to over- or under-curing your parts. Both over- and under-curing your parts will affect adhesion and can make the coating very brittle. Since these conditions do not occur when you run your process normally, this problem will appear to go away. Therefore, my first inclination and joke can be the root cause of your problems. —N.L.
Q: Can Class A terminology be used for a part that is to be powder-coated? I know that parts can be prepped and cleaned to be Class A, but can they expect the actual paint to have any imperfections at all? As far as I know, this terminology can’t be used for powder because it can’t be controlled as easily as liquid paint can. L.E., San Jose, Calif.
It depends. The only two things that affect powder coatings more than liquid coatings are orange peel and film thickness. Anyway, what’s a Class A finish? Automobiles demand a Class A finish. Well, powder is used on most automobiles, and some very expensive ones at that, so where’s the issue? Film build isn’t as tightly controlled in powder coating as it is in electrocoating, but it can be as tight as a tenth of a mil. Dirt is dirt, and defects are defects, no matter what the coating. Dust can be a slightly greater problem for powder, but you can control that, too, unless you are trying to operate on the cheap. So, what’s the problem? —G.T.
Q: I’ve been coating automotive wheels for years, mostly older steel wheels from classic car restorations but also newer wheels. I’ve been hearing lately that aluminum alloy wheels shouldn’t be powder-coated because the curing temperature and dwell time make the aluminum wheels brittle. I have also seen feature articles in the magazine about companies coating wheels but no mention of this problem. Is this true and are there other coating methods that can be used to overcome this problem? K.W., Westminster, Md.
Some aluminum wheels are made from tempered aluminum alloys. The most common type is 6061 T6 aluminum. The T6 represents the temper and is used to increase the hardness and strength of the aluminum alloy. Temper can be annealed (softened) under heat above 300°F. Therefore, great care must be exercised when powder coating aluminum wheels with tempered alloys. Always use powder coatings that have good cure characteristics at 300°F and don’t cure them above this temperature. —N.L.
Q: We’re powder coating 18-gauge Galvannealed material. We’re cleaning with a five-stage washer and powder coating with a black wrinkle epoxy. We’re seeing bubbling and fingerprints coming back through the powder. We use infrared heat and are manually spraying the powder. Is there something we can do to prevent this? This material has given us trouble in the past. K.M., Cedar Rapids, Iowa
The bubbling is a problem with galvanizing of any type. Often, a little preheat will drive out the gases, and then you can powder coat after cooling the part to ambient. The fingerprints? Well, check closely before washing to see if someone is handling the parts with heavy grease or oils on their hands or gloves to determine if the washer is having a problem removing the residual. Maybe the sprayer is turning the parts with his hand and leaving a print. If he has to do this, give him a wire hook to touch the parts so that he doesn’t have to use his hand. —G.T.
Q: Is it possible to apply a powder coating over satin chrome or high-polished chrome-plated parts? If we could, it would save us a lot of money in rework and scrap costs. However, some of our parts are hand-operated, and we don’t want our decorative finishes to start peeling after several years. R.C., Steeleville, Ill.
I don’t know of any way you can powder coat over chrome without a high risk of peeling in the immediate or near future unless you abrade the surface with steel wool, 3M-type pads, or similar materials. The hard chrome surface won’t allow the coating to get a grip. Satin chrome might extend the time before peeling commences. I’ve never tested adhesion to satin chrome, so I suggest you try some samples before you go to production pieces. Acid etching the chrome is ineffective unless the acid is very strong, and then you have a great risk of damaging the chrome. It’s very risky to expect the powder coating not to peel. You have no control over how the part may be used by the customer. —G.T.
Q: We powder coat in two facilities, and our safety director is directing us not to use compressed air to clean off ourselves after applying powder. I guess OSHA (Occupational Safety and Health Administration) will site us for this practice. I’m curious how others in our industry are cleaning off afterward. I have an air-operated vacuum, but it’s so powerful, I’m afraid it would be more dangerous than compressed air. M.G., Iron Mountain, Mich.
There is an OSHA-approved tip for an air hose that restricts output pressure and has relief holes that will divert the air in case the user gets the tip too close to something, namely the body. These are used just about everywhere and have been around for years. The automobile companies have been using these tips for many years, and they are extremely safety conscience. Your safety director should be made aware of these devices so that you can go about your business. Using a vacuum with the suction restricted is a possibility, but vacuuming clothes is awkward and time-consuming. I hope the safety person listens to you when you tell him about this. —G.T.
Q: We have an aluminum frame that has gold chromate conversion coating under the powder and masking lines. We have to re-powder coat the part. The part was powder-coated twice, and the customer doesn’t like the finish. The part came out with heavy texture and a rough finish. We had to sand and re-powder coat, and the gloss ended up being dull and sandy feeling. L.B., Springfield, Mass.
As you probably know, at least by now, heavy powder coating will result in pronounced orange peel, which some people don’t like. Determine the extent of the orange peel the customer will find satisfactory. That will become your standard. Then, you should show your customer what a second coat looks like to see if he objects. Your customer must be aware that if the second coat isn’t to his liking, the part will probably be scrapped. I hasten to add, however, that if you have or have access to chemical stripping, you might be able to strip off the old powder and then reprocess the parts. Most likely, the chemical stripping will affect the conversion coating, and it will need to be reprocessed through the bath. You should discuss this with the conversion coating supplier. I’m not sure why the reworked part was dull and rough. That would depend on how you sanded it and how you cleaned it after sanding. If you sanded through the chromate treatment, then you set up the aluminum to begin its self-protecting patina, which could cause the rough finish–and will cause a loss of adhesion at some point. —G.T.
Q: We recently coated machined aluminum parts with many holes of different sizes with a flat, beige polyester powder. The surface immediately surrounding the plugs was bumpy. The second time we coated this same part the customer purchased the plugs from a different vendor and cut the plugs flush with the surface, but the results were the same. We cure at 400°F for 15 minutes; we’ve had the problem with other parts and think the plugs are made from silicone. Is there a reaction between the plugs and the powder? How can we correct the problem? Thank you in advance. P.L., Cleveland, Ohio
All masking devices (plugs, caps, tape) must withstand the cure temperatures for powder coatings. In most cases, the plugs are made from silicone materials for this purpose. Silicone in solid form isn’t a problem with any coating system. Airborne silicones are what caused fisheyes and other finishing problems.
The bumpy and rough surface around the plugged holes is due to low film build (coating thickness). The space between the end of the plug and the part is very narrow, making coating this area difficult. Trimming the plug to reduce the shielding from the masking device can help with this issue. Ensuring proper coverage in this area will also eliminate this problem.
Finally, the machined part may have some machining fluids/coolants inside the hole. If you mask the part before pretreatment/cleaning activities, this fluid/coolant will stay in the hole. When you cure the powder coating this liquid will vaporize and push this gas past the plug mask and blow off some of the powder coating in the area. You can try fixing this by cleaning and drying the part first, then inserting the plug for masking. —N.L.