Weekly Questions and Answers

By George Trigg, GRT Engineering
and Nick Liberto, Powder Coating Consultants

Welcome to Powder Coating magazine's Weekly Questions & Answers column. 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.



8/24/2015 - Q: I apply a TGIC-based (triglycidyl isocyanurate-based) polyester powder coating over electrocoat (e-coat) on a carbon steel substrate. The e-coat meets automotive specifications as this is the primary customer of my e-coat vendor. My problem is that the parts, which are mostly flat, have been coated for about 1 year, and the powder coating (not the e-coating) is losing adhesion and peeling off. What’s the proper method of cleaning and treating the e-coated parts before application of the TGIC-based powder? My e-coat vendor and powder coater aren’t the same, and I’m trying to find the answer as I'm sure one will point the finger of fault at the other. F.L., Johnson City, Tenn.

A: There shouldn’t be any issues with coating over e-coat, as if you didn't know. Most e-coat applications are designed to have some type of top applied, be it powder or liquid. My first and foremost question would be about the gloss of the e-coat. If you can get your hands on a gloss-meter, take a reading on the e-coat. Then, spray out a sample of the powder on a test panel and check adhesion. Do the same with an e-coated panel and then a powder-coated panel. I know this sounds redundant to you, but humor me. Moreover, check the material safety data sheet (MSDS) to see if the gloss rating is listed. After you know all of this, ask your e-coat supplier to visit your plant. Show the supplier the test panels and the rejects from your customer. My guess is that the material is too high in gloss so that there is no inner-coat adhesion. In my experience with this coating technology, the cured e-coat was sent directly to the powder system without washing or conditioning of any kind. Washing the e-coat won’t normally cut the gloss or condition the surface to accept and adhere to the topcoat. Sanding might get you out of trouble in the short term. If it does, that likely proves that the e-coat is the issue. —G.T.


8/17/2015 - Q: I currently have a set of very expensive magnesium wheels that were powder-coated white by a previous race team. I would like to refinish the wheels myself. Can you recommend a blast media and powder-coat process including bake times and temp for a backyard mechanic like myself? I subscribe to your magazine and would like to get started with this project but more on an entry level without investing in commercial-grade equipment. I would like to show off the car with the statement that I restored the wheels myself. P.F., Dearborn, Mich.

A: I think if I had some very expensive wheels for a show car, I would want them recoated by a professional shop. But that's just me. You should try a plastic media, or maybe glass, not metal. The media should be very small so as not to affect the surface appearance of the wheel. The bake time and temperature is the responsibility of the powder supplier. You don't want an extremely high bake temperature because of the potential effects on the magnesium. —G.T.


8/10/2015 - Q: Is there any special paint I can use to repair chips and scratches on oil-rubbed bronze powder-coated aluminum in the field? R.B., Addison, Ill.

A: Touch-up coatings are available for most powder coatings. These come in a variety of types (spray cans, liquid pens, small bottles with brushes, etc.). Selecting which touch-up device depends upon the amount of repair that is necessary. For instance, very small chips can be touched up by using the pens or small bottle/brushes, while larger areas require spray cans.

Having said all that, touch-up on rubbed bronze may be very difficult, depending upon how the look was first created. If it’s a single-coat process, then any touch-up paint provided by the supplier will work. However, if it’s a multi-step process, using a combination of plating or a different base and topcoats, performing touch-up will be near to impossible without having a noticeable repaired area. —N.L.


8/3/2015 - Q: We’re wondering if anyone has had a positive experience in high-pressure power washing of paint conveyor hooks and chain? If so, please pass on your solutions. M.L., Wyoming, Mich.

A: Be assured that high-pressure washing won’t clean powder coatings from hangers and hooks if they've been through the oven. As for the conveyor chain and rail, high pressure will clean off the uncured powder, but I wouldn't advise doing this in or near the spray booth because of the mess. As for cured powder, I'm not aware of any high-pressure wash system that will remove it. —G.T.


7/27/2015 - Q: What’s the best way to get powder to stay in tight corners, and what causes the powder equipment to shoot a glob of powder out of a gun every so often? K.G., St. Louis, Mo.

A: The powder isn’t going into the tight corner because of the Faraday cage effect. From an electrostatic point of view, the nearest point to the gun electrode will be the point to which the gun discharges, or grounds. In the case of corners, the nearest points are the sides of the part you’re spraying. To complicate things further, when you are aiming the spray gun into a corner, the atomizing air tends to blow away any accumulated powder, making the situation worse. There are various ways to improve the situation, from the no-expense side to a change in application equipment. Reducing the air and the voltage is the first and easiest way to get around the Faraday cage effect. You can also back the gun away just a tad, especially if it's an occasional issue. If you spray a lot of corners, then you may want to look at a tribo-charging gun. As for the clumps coming from the gun, there can be several reasons for this. One is moisture in the powder usually caused by not having a working air dryer on the compressed air system. High humidity in the atmosphere can cause lumping when the hopper, using good dry air, is turned off for a time, and the conditions in and around the coating application area are moist. This moisture will agglomerate the powder (make it lumpy). You may have some powder plugging in the pump, the feed hose, the gun, or the gun tip. Any or all of these will emit lumps of powder, which invariably make their way to the part and cause a defect in the coating surface. From a performance standpoint, the lumps don't really hurt anything, but they look bad and indicate a lack of maintenance to the system in most cases. —G.T.


7/20/2015 - Q: We’re currently powder coating a compressed metal seat and can’t get the powder to cover the runs in the metal. We’re using a polyester powder that should cover it. Any suggestions? We need help! J.L., Mt. Clemens, Mich.

A: If you have imperfections in the metal, such as holes or crevices or dents, or if you have ridges, pimples, or whatever, don't expect the powder to act as a metal repair kit. Any coating--powder or liquid--will only follow the contour of the part. So, if you have a crack, for instance, the powder will flow into the crack, but it won't fill it and then come out smooth. You’re going to have to prep your surface to smooth it out first before you apply the finish. —G.T.


7/13/2015 - Q: Like any shop, we do some recoats when the first coat is rejected, be it some pickup or light paint. When we do recoats, we handle it with gloves, sand it, blow off the piece, and wipe with isopropyl alcohol. When we try to repaint the piece, it appears to have streaks where the powder won’t stick (like bare spots where it was touched by hands with latex gloves or wiped). I have had this problem before, and it stopped when I heated the product up after handling and then waited for it to cool before powder coating it. I’ve tried to suggest this to my supervisor, but he thinks it’s something else and not from handling the piece. But I can see clear finger marks, and I even wrote numbers on the product with the gloves, and the paint wouldn’t sit where I did this. I’m just wondering what this is and if I’m right in saying to heat it up, or rinse it in cold water and dry it or let it sit after handling for a period of time to prevent this from happening. J.M., Langley, B.C.

A: You know the old saying: “Believe nothing that you hear and only half of what you see.” This proverb applies here, as well. However, it doesn’t say ignore what you see. To that end, if you see handprints or fingerprints under the coating, then they’re there no matter what your boss says.

Gloves often give people a false sense of security. The type of gloves and the frequency between changing them are two factors that affect the protection you obtain by using them in the first place. The best gloves are lint-free cotton gloves (preferably white in color). Change the gloves often, especially when the white color is stained or dirty. Lint-free cotton is comfortable and ensures that the operator doesn’t have an allergic reaction or deposit lint onto the surface.

Latex gloves have powder to keep them from sticking to each other and to make them easier to put on your hands. This powder can be a source of contamination. Besides, latex gloves are very uncomfortable to wear, especially when it’s warm and your hands are sweaty.

Recoats need to be handled differently than virgin parts. The surface must be completely cleaned after the defect has been removed (by sanding or stripping). Often, the entire surface needs to be scuff-sanded to ensure proper adhesion to the topcoat. All the dust must be removed and the surface completely cleaned before applying the next coat. Don’t use tack-rags, as they can often leave a residue on the part surface that will prevent the powder coating from adhering to the part (not a problem with liquid coatings, as the solvent will penetrate this contaminant).

Denatured alcohol is good, if you change the rag and solution frequently. Using a dirty rag and contaminated alcohol will just reapply the soils onto the part. Using the alcohol directly from the can on a clean rag is best, changing the rag areas often to ensure cleanliness. —N.L.


7/6/2015 - Q: We have a new system that we’re running 8 hours a day. We apply the powder with 10 automatic spray guns. When we start to touch up spots that the automatic guns miss, we see the powder taking on some kind of charge and leaving round spots. It acts as if it has static and wants to drop off the parts. The system is grounded good. Could it be humidity in the air? We have a room that is 40 feet by 70 feet and is kept at 72°F. This is where our booths are at and the powder is stored. We also are running all of our parts in a blaster wash station, a dry-off oven at 300°F, then a cure oven at 400°F for 35 minutes. D.S., Assumption, Ill.

A: What you’re seeing is called “kilovolt (KV) rejection.” It occurs when you hold the gun too close to the part or when you spray too much powder in a given area. I would expect the film build in that area to be quite heavy. A case can be made for doing the touch-up first, then spraying with the automatic guns. You need to run some parts with the automatics only to see where the light spray areas are located. Spray a part with no reinforcement, take a film thickness measurement, and observe the light areas. This will give you a hint as to where the manual application needs to be, and how much. Maybe with some judicial testing you could re-align the automatics and do without the manuals. —G.T.


6/29/2015 - Q: I’ve been trying to perfect my gloss-black parts by using less powder, but my parts still come out with orange peel. I've mentioned to my boss that maybe the curing time is too long and the temperature is too hot. Could you please help? D.A., Wilmington, Dela.

A: In your case, the thinner the film, the more prominent the orange peel. You’ll never be rid of orange peel in powder coating, it's just a matter of how much orange peel you’re willing to put up with on the parts you’re finishing. Your powder coatings supplier can help you diminish the orange-peel effect by supplying you with a fine-grind powder. You’ll pay more for it. You also may have fluidizing and atomizing problems, which can mean a lot of reclaim. As far as the time-and-temperature formula, you should be curing to the powder manufacturer’s specifications, so you really don't have any latitude that way. —G.T.


6/22/2015 - Q: I’m just now getting ready to buy some powder coating equipment for our business but haven't yet found a forum that answers a nagging question I have. Can clear powder coating be done on aluminum on which I have already painted some custom designs using urethane automotive paints? I want to replace the urethane clear coat I’m currently putting on the artwork with a powder coat clear to make the pieces more durable. The paint is stuck very well to the aluminum, but I just wonder if the powder coating process will damage the detailed paint work that is already in place or if the clear will even stick to the painted areas. I’ve also read that there are various clears, such as 85 percent, 95 percent, and so on, but I want it as clear as possible. P.C., Johnstown, Colo.

A: There is no way of knowing beforehand if you’ll have problems coating your artwork with a clear powder coating. These powder coatings are often used over liquid basecoats. However, the liquid coating may cause solvent pops in the powder coatings, depending upon the solvent content and the drying time. The liquid paint may discolor during the powder cure process. My best advice is test a few powder coatings over some scrap pieces to verify results before you destroy a unique piece of artwork. —N.L.


6/15/2015 - Q: Who has the closest looking powder to a true chrome finish? I manufacture automotive aftermarket accessories and would like a good alternative to chrome-plated products that would be more environmentally sensitive. D.L., Commerce, Calif.

A: Most powder coatings suppliers offer the material you’re looking for. Some of this stuff looks really good, actually defying you to distinguish it from real chrome. Don't accept just anything they send you. And don't be satisfied with their panels. When you see some color panels that look good, ask the powder supplier to give you a small sample to spray it out yourself. Some of this material looks great but is a production pain to apply. If you need a list of suppliers, go to www.pcoating.com and click on Online Buyer’s Guide. —G.T.


6/8/2015 - Q: I have metal boxes powder coated various colors. Can I recoat them in a darker color? Is there something I should do first for the powder to adhere? L.D., Ludlow, Mass.

A: You can powder coat them any color you wish. Be aware of some of the following issues. If the original coat is a high-gloss, you’ll need to scuff-sand the entire part. If the original paint is a low-gloss, the only thing you’ll have to do is make sure the surface is clean and free of dirt, oil, and stuff like that. Of a bit more subtle nature is the fact that the second coat will have a fairly pronounced orange peel. It's just the nature of powder coating. Also note that the second coat can give you a total film thickness that could interfere with mating surfaces or other pieces that have to fit together with a tight tolerance. The second coat will be of slightly lighter film build because of the insulating factor of the first coat. So, if you're powder coating a different color, be sure to inspect all appearance areas for good hiding of the first coat. —G.T.


6/1/2015 - Q: We powder coat office furniture (filing cabinets) and always use a hybrid powder coating for interior use. Sometimes after some of the parts have cured, there will be a flow from the powder that bridges the gap where the two pieces of steel are welded together. I notice it more if the film build is getting heavy, up around 3.0 mils to 3.5 mils. Sometimes, it's on a recessed area where two separate pieces meet, and sometimes, it's a flat surface where two pieces are welded together on a small offset. It usually happens when we're coating white or a light silver where we require a heavier film build.

Second, how opaque should a light-colored hybrid powder be? I know it depends on how light the color is, but I'm talking very light, about the color of printer paper. P.L., Pembroke, Ont.


A: Thanks for reading the column. I hope we do some good and help reduce the frustration that comes from operating a finishing business. If I have the picture of your problem somewhat correct, then maybe I can offer the following. I see the likelihood of two issues. If you have any cracks or irregular surface joints, the powder won't bridge them. Instead, it’ll leave a line or a void. If you get lucky, heavy film might camouflage it. Like any paint though, powder isn't designed to cover even the smallest of gaps. In fact, the powder will tend to have edge pull and will pull away from the edge of a crack or irregular surface. You can have the powder modified for less edge pull, but it won't fix it so that it's never there. The other issue is welding. Welding leaves a residual. The severity of it depends on the kind of welding, which has to be sanded or etched or ground away for the paint to get any adhesion. So, if you're seeing pull-away from the welded areas, I suspect that the immediate surface of the weldment isn't properly cleaned. By the way, laser cutting will do the same thing.

As for your second question, opacity is the capability of a coating to cover or hide, especially as in the covering of a substrate or previous coating. OK, now if I interpret your question correctly, you're wondering how well a very light color should hide the surface to which it's applied. It should hide the substrate no matter what the color/pigmentation is, providing the powder is applied at the manufacturer's recommended film thickness and providing you have at the very least a continuous film. A continuous film won’t hide the substrate, but it will be well below whatever film thickness you need and is recommended. At 2.0 mils, for example, you shouldn't see any substrate. If the powder is designed to cover at 1.5 mils, you shouldn't see any substrate, whether the tint/color is white, black, or in between. It shouldn't make any difference if it's a hybrid or a straight material. —G.T.


Further reading on the problems discussed in this column can be found in our Article Index and Bookstore.

George R. Trigg is president of GRT Engineering, 6314 Hughes Road, Prospect, OH 43342; 740/494-2496. He has been involved in the powder coating industry for more than 38 years. He holds a BSBA degree from Muskingum College, New Concord, Ohio. His email address is molly95@earthlink.net.

Nick Liberto is president of Powder Coating Consultants (www.powdercc.com), a division of Ninan Inc., 1529 Laurel Avenue, Bridgeport, CT 06604; 203/366-7244. He has more than 3 decades of experience in the powder coating industry. A registered professional engineer in Connecticut, he holds a bachelor’s of science degree in mechanical engineering with a minor in physics. His email address is pcc@powdercoat.com.



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