What is Causing Streaks?

Q. We apply a medium-gloss gray powder at thickness of around 3–5 mils. Some of our panels have a large surface area and we sometimes get some streaks on the parts and areas that are darker than others. These streaks are not visible on the part surface before coating, but they are clearly visible in the dry coating film before cure and after the coating is cured. The stripes do not follow any particular pattern and some parts do not have any at all. All of our application is done manually, and the operators cannot figure out what causes this striping. Any advice you can give us on the cause of the streaks and how to avoid them would be appreciated. T.R.

A. The stripes you see in the coating are most likely caused by variations in gun-to-target distance. When the gun is held too close to the part it will cause excess film build and texture. The ionization in a localized area increases due to the high current draw resulting from lowered resistance between the gun electrode and part. The variations in electrostatic behavior and the resulting changes in texture and film thickness are causing the striping. The proper gun-to-target distance for manual application is typically around 6–8 inches. If the operator must get closer to feed powder into a Faraday area, be sure to limit the current draw with a factory preset or turn the voltage down to prevent excess current draw. Gun strokes should be smooth, square to the target and consistent across the entire surface. Gyroscopic action (moving the gun in irregular arcs) is not recommended. Teaching the operator to control the gun stroke at a consistent distance from the part should take care of the problem.