Causes:
Application of a colored coat on top of ground coat that has been sprayed dry.
The air bubbles that form during the application because of high viscosity, low air pressure or because of high ambient temperature that lead to a very quick drying.
Old pinholes that were not sanded well.
Poor application of the putty.
Fiberglass bodies.
Insufficient mixing of polyesters.
Insufficient priming of polyesters.
Preventive measures:
Avoid spray dusting of the initial coats.
Use recommended viscosity (thinning ratios) and air pressure.
Sand well ground coats.
Use correct technique when applying putties. Hold the knife at 60-degree angle in order to avoid rolling the putty and allowing the penetration of air bubbles.
Mix polyesters thoroughly.
Use polyester spray filler (574 with H353 hardener) whenever possible.
Ensure sufficient filling of polyesters.
Remedy:
Remove damaged finish.
Sand and apply polyester spray filler (574).
Prime and paint.
Note:
You should never try to fill the holes with successive coats of primer filler.