New paint:
Dirty spray gun.
Incorrect clear coat and/or hardener.
Old paint:
The clear coat is too thin.
Contaminated hardener.
Incorrect clear coat.
Effects of corrosive environmental conditions.
Prevention:
Ensure that hardener containers are tightly sealed after use.
Follow the recommendations in the technical data sheets (TDS).
Use the recommended hardener.
Correction:
Allow the paint surface to dry properly. Sand and repaint.