Causes:
Excessive coat thickness of the basecoat.
Intermediate and final flash off times of the basecoat are too short.
Delay between the final coat of the basecoat and the 1st coat of the clear coat.
Wrong mixing ratio of clear coat/hardener.
Basecoat surface is contaminated (finger marks, sweat, dust, etc.).
Preventive measures:
Do not exceed 3 spray passes of basecoat.
Observe intermediate and final flash off times.
Do not wait too long after spraying the final coat of basecoat. (If a long wait is unavoidable you should sand the basecoat and respray).
Use the recommended hardener in the correct mixing ratio.
Avoid any contamination of the basecoat before spraying the clear coat.
Remedy:
Sand and repaint