Pinholes in Powder

Q. We have a problem with a concentration of fine, dense pinholes in our powder film. The coating is a urethane-polyester that is applied over a part that has been electro-coated. The substrate is sheet metal. The problem is much worse when the powder film is heavier. The e-coat is cured at 165°C (329°F) for 20 minutes, and the powder is baked at 195-200°C (383-392°F) for 15-18 minutes. H.P.

A. This problem could come from different issues, so I will list the likeliest causes. Check with your supplier on the e-coat. Your cure cycle may be OK, but it is a little low for some products. Make sure the e-coat is fully cured. Polyester urethane powder contains a missive component called e-caprolactum. When the e-cap evolves during the cure cycle it may create small ruptures in the powder film. Thicker films are more likely to have problems, and this could be the root cause of your problem. Make sure you use a powder that is compatible with the e-coat. Try different powders at different thickness to determine if the powder is the problem. Contamination also is a possibility, so do not overlook some source of oil or similar problem. Make sure that the e-coat is not contaminated before application of the powder. There could be other causes but these are the most likely places to look into.