Answer
Block sand primer surfacer with 320, then refine with 400 or 600 before paint.
Why it happens
Primer surfacer needs leveling with a block and then finer scratch refinement before topcoat.
Recommended grit
Use 320 for leveling, then 400 or 600 for final paint prep.
Wet or dry
Dry for block sanding; wet can be used at 600 where the paint system allows it.
Success check
The primer is level, smooth, and free of visible scratches before paint.
What to do
- Let primer surfacer cure.
- Block sand with 320 to level the surface.
- Refine with 400 or 600.
- Clean thoroughly before paint.
Avoid: Do not paint over deep 320 scratches without refining them.