Issue link: http://www.e-digitaleditions.com/i/901126
POWDER COATING, November 2017 25 Covering power of powder coatings Q We're having some issues with parts that have to be spot-welded. The process creates a mark on the metal. This drag- ging 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 charac- teristics. 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 im- prove your manufacturing process to eliminate surface defects on the metal be- fore painting. Another is to change your coating to better hide the defect. If im- proving the manufacturing process at the source isn't practical, then adding an- other step to sand the surface smooth be- fore powder coating is another choice. Selecting a powder coating with better hiding power can be as simple as chang- ing the gloss or increasing the orange peel (up to and including textures, wrinkles, and so on) to hide the defects. —N.L. Getting rid of fuzzy growths in the cure oven Q We powder coat ATV frames. We've been having problems with some type of growth in our curing oven. The growth is orangish brown and fuzzy. It seems to grow at the entrance and exit of the oven or exhaust fans. It doesn't grow in the center of the oven. We are coating on a power-and- free conveyor and run our oven at 390°F. Can you help us with this mys- tery growth? Thanks. S.S., Rome, Ga. A I've seen similar deposits in liq- uid paint ovens and in electro- coat ovens. They usually come from the byproducts of the combustion process. Powder has virtually no byproducts, so it wouldn't be caused by the curing process. You might get a buildup on the entrance as powder drifts off of the part and is caught in the airflow. This could also apply to the ex- haust fan but shouldn't be the case at the exit, unless the exit and entrance portals are the same. I think your powder coatings supplier may be able to answer and correct this problem. If it really bugs you or is causing a problem, take a scraping and send it to a lab for analysis to see what it is. That way, you may at least have some ammu- nition on your side of the table. —G.T. Removing powder overspray from car upholstery 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. Meeting a gloss spec with manual spraying Q We're applying a textured white powder, and we're fac- ing some issues in order to meet the gloss. Our spec is 3.0 to 5.5 on a 60-degree glossmeter. The cure sched- ule for the powder is 12 minutes at 350°F. We have succeeded in achieving the gloss when applying the powder in a manual spray-to-waste booth where we got values between 4.0 and 5.0. How- ever, when we start applying the pow- der in the automatic cyclone booth with a reclaim system, the gloss value drops to 2.5 to 2.9. The values of film thick- ness are about the same 4 to 6 mils. We don't know if there is anything on the reclaim that might be affecting the pow- der and killing the gloss. I would appre- ciate any advice to get the right results with the automatic equipment. F.G., Guadalupe, Nuevo Leon, Mexico A The reclaim material is causing your gloss change because the oversprayed powder (reclaim) has been worked. This means it's a smaller grind size and has been abraded through the reclaim system. As a result, the particles no longer have nice, sharp edges. These particles are affecting the appearance of the finished coating. As you have found, straight virgin material gives the results that you need. That response is easy. Fixing it isn't. Your materials supplier should be in- volved in helping you make some ad- justments to the material. But don't bet the farm on that. I'll say the obvious: You need to minimize the overspray so that it's just a small part of the sprayed material. Make sure you don't have any more guns spraying than necessary. And make sure the gun settings don't give you too much material. —G.T. Powder coating anodized aluminum Q What types of problems can occur when powder coating over an anodized aluminum finish? S.D., Dalton, Ohio A A lot of problems can occur, but most of them can be pre- vented with some planning. The obvious prevention is to make sure the part is clean and free of any marks, such as fingerprints. I can only assume that you want to spray a clear powder over the anodized. Clears are very touchy. They're film-build sensitive, which will show immediately when the parts are cured. I'm not aware of any particular issues such as cratering or pull-away though. —G.T. Powder coating a missed area on a part Q I put powder on a part to cover a missed area, and the part was too hot and burned in. Can I wet-sand the part and then clear it? Any suggestions without start- ing over? M.T., Helper, Utah