Blistering on Recoat

Question:

We paint our steel enclosures before assembly, and the painted enclosures are stored. We often have to rework these enclosures because of paint chipping and scratching during handling. We are getting blistering of paint when we re-apply paint over ASA 61 gray baking enamel. Before repainting we apply a tie coat. We then re-apply our ASA 61 gray baking enamel.

The finish comes out like a reptile. The epoxy is an air dry. What is the best tie coat? We cure at 315-330F. Our line runs at 10 ft/min. Thanks. J.B.

Answer:

Obviously, the tie coat you are using is not the best. Otherwise, there would not be a problem. First lets talk about tie coats. They are used when there is some incompatibility between the original finish and the new finish. Since you are re-applying the same finish over the original one you should not need a tie coat unless there is a slip agent in the original finish. If that is the case, you may be better off removing the slip agent.

One other important thing about tie coats is compatibility with both the original and new finishes. The solvent in the baking enamel could be attacking the resin in the tie coat, causing the film to soften. Upon baking the re-applied enamel, the solvent is coming out of the tie coat, causing the newly applied topcoat to blister, mud crack, alligator or whatever it is actually doing. One solution to this is to make sure the tie coat is completely cured before re-applying the baking enamel.