Splotchy Appearance in Coating

Q. We use manual spray guns to apply a textured powder over a cast part, and the color that is the biggest issue is black. The problem is that the finish has a dark/light, splotchy look to it after it is cured. You can see the color variation at the booth, and it is still there after cure. Any suggestions on how we can get better appearance? J.P.

A. The appearance you describe as splotchy is most likely caused by the gun settings and application technique. The flow rate is the place to start. Adjust the powder flow rate and secondary air controls so that the powder is smooth and free from any surges at the tip. Adjust the air volume up and/or powder down until you are free of any surfing. Make sure the amperage and voltage are at the optimum level. Start with low voltage (65 or lower) and low amperage (20 or lower) and adjust down, if necessary, to avoid back ionization or up to improve charge efficiency.

Keep in mind that charging efficiency is partly related to gun-to-target distance, so you do not need high voltage for manual application at close range. Keep the gun at a consistent distance for the part and stroke the same way with slow, methodical passes in line with the length of the part. Match the pattern up to the size of the part. Avoid radical moves of the gun that change the pattern or the distance from target, the so-called “gyroscopic” style of coating. The combination a smooth pattern, low amperage and voltage, and a consistent stroke should correct your problem.