Swelling as bubbles or pimples in the film.

Cause
A. Applying over water, grease or oil.
B. Rust on the substrate.
C. Exposure to the high humid condition.

Repair
Sand the swelling area or peel off to the substrate and refinish.

Prevention
A. Wash the substrate clearly.
B. Have enough drying time between coatings.
C. Remove the moisture in the air hose.