Paint Blistering On Plastic Bumpers

Q. We are a painting subcontractor for automotive plastic parts—bumpers, door handles, side mirrors, and so on—here in the Philippines. We use specified paints for painting bumpers. Our process is to apply base color paint (polyester based, the same paint the auto manufacturer is using on its car bodies) and finish off with 2K polyurethane clear coat. All paint applications are wet on wet. The scheduled baking condition is 30 min at 85°C, as specified by the paint supplier.

The bumper parts are supplied to us pre-primed by our customer. We believe the primer is applied by our customer’s part supplier from another country. We scuff sand the surface for better adhesion and to remove seeds and dirt on the surface.
After about three months, blisters appear on the surface of the part. The size of blisters vary from 1–4mm in diameter. The problem appears on a portion of the parts only, not on the whole surface. Blisters appear both scattered and grouped together. The problem is on the rear bumper only. The front bumper has no problem at all. Prior to painting, we noticed that the primer on the rear bumper is thinner than the front. Could you please help us find the cause of the problem? D. N.

A. The greatest cause of paint defects is poor surface preparation. That being said, I am reminded there are a number of contributing factors. The cause of the problem may not be at your plant. It could have been caused at the part supplier’s plant. There should be no seed nor dirt in the primer film. Since the problem is with the rear bumper only, it may be related to the way the parts are hung or racked during pretreatment and priming. Furthermore, the rear bumper could have been fouled by airborne contaminants during and after curing. To absolve your operation from blame, I suggest you fully investigate the cause of the problem. For example which coat is blistering, primer color coat or clearcoat? Which coat is on the underside of the blisters?