Answer
For fiberglass, shape with 120 or 180, prep with 220 or 320, and wet finish with 600+.
Why it happens
Fiberglass work often needs a grit that levels repair material first, then finer grits for coating or polishing.
Recommended grit
Use 120 or 180 for shaping, 220 or 320 for prep, and 600+ for fine wet sanding.
Wet or dry
Dry for shaping; wet for fine finishing.
Success check
The repair is level and the final scratch pattern matches the next coating step.
What to do
- Clean the fiberglass area.
- Use 120 or 180 for rough shaping only.
- Use 220 or 320 before primer or coating.
- Use 600 and finer for wet finishing.
Avoid: Do not use coarse grit as the final prep before paint or gelcoat finish.