Ask the Expert Question-and-Answer Archive

by Ron Joseph

July, 2003

Paint Failure Over Zinc-Rich Primer

Q. I was confused by a paint failure and we don't know how this phenomena occurs. We apply the polyamide cured epxoy high-build intermediate coating on the primed steel structure. The primer is zinc-rich epoxy primer and the application procedure complies with the requirement of application instruction. After coating the welding seam by brush, we apply the intermediate coat by airless spray. After the application, we found the pinholes on the brush application area while the airless spray area is good. How can we avoid this problem?

A. My first thought is that there was too much solvent in the coating and when you applied it by brush the faster evaporating solvents were still in the coating. When you applied it by airless spray the faster solvents evaporated while the coating was being atomized.

My second thought is that you spray-applied the epoxy coating over the brushed coating, and much of the solvents from both coats of epoxy were entrapped and didn't get out fast enough. Did you allow sufficient time for the solvents from the brush-applied coating to get out before you spray-applied the epoxy over the brushed coating?

A third possibility is that the brush was contaminated, although this suggestions doesn't sound quite reasonable to me.

Before you go any further, I suggest that you focus your attention on getting all the solvents out of the epoxy. Try applying the coating by brush and then waiting for a few hours before using the spray gun. Perhaps you can speed up the solvent evaporation by blowing air over the surface with a large industrial fan.

Please get back to me and tell me what you find. I would like to know if any of these ideas will solve your problem.