Cause

• The primer coat was not sufficiently through hardened for sanding
• Grit or coarse dirt particles have caused scratches during sanding
• Too coarse sanding material
  

Prevention

• Allow sufficient through hardening before sanding
• Before sanding remove all dirt carefully from substrate
• Use fine sanding paper of 320 p
  

Remedies

• After it has hardened, sand the topcoat smooth using the correct grade of sandpaper and, if necessary, re-apply the topcoat once more