Answer
If primer scratches show, refine with 400, then 600 before topcoat.
Why it happens
The primer was not refined enough or the previous grit marks were too deep.
Recommended grit
Use 400 for primer prep, then 600 for smoother topcoat prep.
Wet or dry
Dry or wet only if the primer system allows it.
Success check
The primer has a fine even scratch pattern before paint.
What to do
- Inspect scratches under good light.
- Sand with 400 until the scratch pattern is uniform.
- Refine with 600 before topcoat.
- Clean dust completely before paint.
Avoid: Do not topcoat over visible deep primer scratches.