Answer
For automotive primer before paint, use 400 or 600. Use 800 for finer wet prep when the paint system requires it.
Why it happens
Primer needs to be sanded fine enough for topcoat while staying clean and even.
Recommended grit
Use 400 or 600 before paint. Use 800 for finer wet prep when the paint system calls for it.
Wet or dry
Dry or wet depending on the primer and paint system.
Success check
The primer is smooth, clean, and evenly dull before paint.
What to do
- Let primer cure as directed.
- Sand evenly with 400 or 600.
- Use 800 for finer wet prep when needed.
- Clean thoroughly before paint.
Avoid: Do not paint over coarse primer scratches.