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Description of problem:
The sanding marks underneath the topcoat show through.
Causes:
Ground coat is too thick.
Insufficient drying time before the application of the topcoat.
Sand paper used is too coarse.
Substrates are soft, not dried enough or sensitive to the solvents present in the topcoat.
Too low film build.
Preventive measures:
Do not spray thick substrates. They may be dry on the surface but still soft on the inside.
Leave enough flash off time between coats.
Use correct sandpaper grade.
Use solvent test to identify soft substrates (i.e. T.P.A).
Apply normal film thickness of the topcoat.
Remedy:
Depending on the seriousness of the problem, either sand lightly with P1200, use rubbing compound and polish, or leave to dry completely, sand and repaint.