Causes:
Use of an inappropriate thinner.
Contamination of the surface before painting.
High air pressure.
Poor extraction and air flow of the spray booth that allows the dust generated by the paint itself to settle on the surface once it has begun to dry.
The paint has dried in the presence of exhaust or industrial fumes.
Poor drying conditions: Temperature too low, high humidity, lack of ventilation.
Mixing ratios of hardener and thinner not according to manufacturer’s specification.
Too many coats being applied.
Not enough flash-off time between coats.
A highly pigmented ground coat was not stirred enough before use.
Use of inappropriate sandpaper grade.
The ground coat was sanded before it was fully dry.
Hardener expired or damaged (by humidity if the can is left open).
Preventive measures:
Use recommended thinner.
Clean the surface well before painting.
Use correct mixing ratios.
Use correct pressure.
Check air flow of the spray booth.
Keep workshop warm in the winter.
Remedy:
Allow the paint to fully dry and polish or respray.