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