Q. We paint ABS automotive parts, but they become brittle after application of a polyurethane enamel topcoat and lose the flexibility they had when they were unpainted. They crack on force application. Do you have any suggestions for solving the problem?—A.M.
A. The solvents in the paint are probably attacking the ABS. One solution would be to apply a barrier coat to the ABS parts before painting with the polyurethane enamel. That would protect the substrate from solvent attack. You also could use a water-reducible primer or a primer with non-aggressive solvents.
Another solution could be to have the supplier of your ABS parts use a solvent-resistant molding compound, but this solution is probably not viable because the ABS molding likely was formulated to meet certain performance parameters.
To prove that the problem is, in fact, paint solvent attacking the ABS, you could soak a small part in the paint solvent for a time equal to the drying time of your paint. If the ABS cracks and crazes, the solvent is attacking it. An alternative would be to paint one section of an ABS part using your existing paint and paint the remaining section using a waterborne paint. If the solvent-thinned paint cracks and crazes, but the waterborne paint does not, the solvent is attacking it.