Ask the Expert Question-and-Answer Archive

by Ron Joseph

September, 2006

Defect of New Paint Over Old Paint

Q: I light sanded a generator with vinyl acrylic paint and repainted with a polyurethane acrylic/mixed with its catalyst. "Paint frying" reaction occured in several places. What could cause this and what is the actual physical reaction that occurs with paint frying effects?  Paint fry is our expression used when a new paint coating is applied onto an already (dried) coating of paint. The initial coating is sanded, blown and tack clothed. When the new coat of paint is applied, it sometimes reacts with part of the original coating and crinkles the paint in various places. Have you seen this occur before? Do you know the cause?

A: Invariably the reason for this defect is that there is an incompatibility between the two paints.  Either the solvents being used in the new paint are incompatible with the old, or the resin of the new paint is sesitive to the overcoating time.   There are some resins which have critical overcoating time. For instance you can apply the new paint within two hours after the "old" one, AND you can apply it after six hours, but you cannot apply it between two and six hours.  This is an incompatibility within the resin.   In your case it sounds as if you have incompatible solvents.  It is likely that the solvents in the polyurethane are stronger ("hotter") than those in the vinyl acrylic

Best wishes,

Ron Joseph