Pops in the Coating

Q. We are having a problem with a large number of fine and very dense pinholes popping up in our powder coating as it cures. We are applying the powder over an electrocoated sheet metal part. The electrocoat is cured at 165°C for 20 min, and the powder is baked at 195-200°C for 15-18 min. We have more trouble when the powder film build is thicker. What could be causing this problem?
A.P.

A. Pinholes can be caused by something on the substrate surface when the powder is applied. Oil or moisture can evolve during the cure cycle and leave a series of round blemishes in the coating. Another possible cause is the powder itself. Some powders, most often polyester urethane, have a missive component that can show the same type of defect as it evolves during the during the cure cycle. This is more of a problem with thicker films. A more likely cause is back ionization. The electrocoat finish creates an insulating layer on the part surface and somewhat limits the amount of current that can be applied to the part surface. If the current is too high it will generate a breakdown of air molecules under the powder film and blow holes in the coating when the positive ion of the air molecule moves out of the coating layer towards the gun electrode. This will be more of a problem with thicker films also. Check your part surface for contamination by trying different washing techniques. If you use a polyester urethane you should keep the film thickness under 3 mils or change to a different powder. Do not get the gun too close to the part. Reduce the micro-amps using the gun features for current limiting if you have them or dialing the voltage down. Always work to keep the film thickness down.