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.

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.

4/18/2016 - 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

A: 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.

4/11/2016 - 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

A: 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.

4/4/2016 - 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.

A: 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.

3/28/2016 - 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.

A: 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.

3/21/2016 - 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.

A: 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.

3/14/2016 - 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.

A: 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.

3/7/2016 - 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

A: 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.

2/29/2016 - 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.

A: 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.

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.

View More Question and Answers |  Submit a question |  Problem Solving Index
Powder Coating Home Page |  Print |  Close Window