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.

7/18/2016 - 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.

A: 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.

7/11/2016 - 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.

A: 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.

7/5/2016 - 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.

A: 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.

6/27/2016 - Q: What temperature should galvanized steel reach when degassing it before powder coating over it? S.M., Louisville, Ky.

A: 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.

6/20/2016 - 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.

A: 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.

6/13/2016 - 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

A: 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.

6/6/2016 - 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.

A: 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.

5/31/2016 - 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.

A: 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.

5/23/2016 - 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

A: 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.

5/16/2016 - 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

A: 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.

5/9/2016 - 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.

A: 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.

5/2/2016 - 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.

A: 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.

4/25/2016 - 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

A: 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.

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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