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point, which will be the sides of a recess or some such area, and in effect will draw the powder to that area while also creating a slight blocking to the deeper part of the part. What you get is a heavy coating in one area and a light coating in the recess. You can improve the situa- tion by reducing the electrostatic charge in the gun. You'll have to experiment a bit with your type of gun. In fact, you may have to turn the power almost off to achieve the coating level you desire. Keep in mind that with each coat of powder, you're creating an insulator that makes the film thickness on the sec- ond pass less than it was for the first pass at the same gun settings. As far as the color change is concerned, different film thickness will cause a color flip with some coatings, especially those that are metallic. I don't know what blue vein is, but my guess is you're asking too much from your powder and gun combination. —G.T. Coating white on one side and black on the other side of a part Q If I want to powder coat parts with two different colors, black on one side and white on the other side, what is the easiest, cheapest, and fastest way to get a quality product? R.S., Mexico City, Mexico A Two-toning obviously can be a problem. Which color you choose to powder coat first is more a matter of choice and circum- stances on your part. You are powder coating the two extremes: black and white. I don't know how you plan to mask the coated side while you coat the second side. This will have an impact on what you do. I would apply the white first be- cause it will be easier for the black to cover the white rather than the other way around. If this is a flat sheet, you might want to picture frame the part to reduce wrap to the other side and to maintain a smoother cured finish. The second color will require a mask that fits fairly tight so that overspray doesn't get onto the coated side. Another way you could do it, which would be less complicated, would be to coat the entire part white and don't use a mask. Then, using a mask, coat the black area. You should test this to see what the cured finish would look like. Actually, you should spray the part white on one side only and then mask and spray the black. Then, inspect the cured finish. If the overspray from the first coat doesn't cause an appearance problem, and the two colors are of the same chemistry, this probably is the way to go. —G.T. Applying a liquid coating over a zinc powder primer Q One of my customers asked me if he can apply a wet paint over a zinc primer (epoxy chemistry-powder coat). I would like to know what kind of impediments can appear if we do something like that. How will the adhesion, surface appear- ance, and gloss of the liquid paint be af- fected? How will the solvents from the liquid paint interact with the primer coat? And how important is the cure temperature for the wet paint in this case? If I need to consider other parame- ters, please let me know. Thank you. G.B., Reading, Pa. A A zinc primer is formulated as a base coat to give the greatest pro- tection to the substrate. It's also designed to receive an over coat. The zinc coating should be a low-gloss mate- rial, and the inner coat adhesion should be good. Having said that, it's always good to spray a sample piece and test it for com- patibility. I feel comfortable in saying that you shouldn't have any issues be- cause of the formulation of the primer. Of course, you should always clean the primed part before applying the second coat unless you are going directly from the primer cure to the topcoat applica- tion. Liquids will adhere well to powder coat- ing, but you can never assume anything so be sure to run the test I mentioned. And always follow the manufacturer's recommended cure times and tempera- tures. The topcoat bake cycle shouldn't affect the primer. Liquid cure tempera- tures normally aren't any higher than powder cure temperatures. Sometimes they are actually lower. —G.T. PC Editor's note For further reading on the problems discussed in this column, see Powder Coating magazine's website at www.pcoating.com. Click on Article Index and search by subject category. Have a question? Click on Problem Solving to submit one. George R. Trigg is president of GRT Engineering, Prospect, Ohio. He has been involved in the powder coating industry for more than 40 years. He holds a BSBA degree from Muskingum College, New Concord, Ohio. Nick Liberto is presi- dent of Powder Coat- ing Consultants (PCC), a division of Ninan, Inc., 1529 Laurel Ave., Bridge- port, CT 06604. Established in 1988, PCC is an independent engineering firm specializing in the use of powder coating technology. Nick has more than 3 decades of experience in the powder coat- ing industry and is a member of many industry associations, including the Application Equipment Technical Com- mittee of the Powder Coating Institute. A registered professional engineer in Con- necticut, he holds a bachelor's of science degree in mechanical engineering with a minor in physics. He can be contacted at 203/366-7244; email pcc@powder coat.com; website www.powdercc.com. 34 POWDER COATING, March 2016