Ask the Expert Question-and-Answer Archive

by Ron Joseph

May, 2003

Powder Coating Defects - Pin Holing

Q. Three questions: What are the general reasons for pin holing in a powder coat? What kind of powder can be used for aluminum alloy wheel as a base coat? And, what causes orange peel on a coated surface? Thank you.

A. I am assuming by the questions you are asking, that you are experiencing pin holes in cast aluminum wheels when you powder coat them. Pin holing can be caused by several different things, in this case I would say that the alloy wheels are porous and the air in the pore is expanding when the parts are heated in the oven. The air then escapes into the powder film when the powder is in the liquid or melted state and as the powder solidifies during the curing process the air then becomes trapped in the film and forms pin holes.

To help reduce the pin holes I would recommend preheating the wheels, bring them up to the cure temp. and hold for at least 10 minutes, so the metal alloy will have a chance to heat up. This will help to chase the air from the pores. Coat the wheels while they are still warm (do not let them cool completely) then subject them to the cure temperatures.

Orange peel is related to a coatings ability, or in this case the inability to flow well. Powder coatings flow much in the same manner as a liquid coating does, when they are subjected to heat. Powder coatings melt and become liquid, causing them to flow and form a continuous film.