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.