Answer
For OSB before painting, use 120 on rough spots, then 180 or 220 before primer.
Why it happens
OSB has uneven wood strands that need light leveling, but aggressive sanding can tear the surface.
Recommended grit
Use 120 for rough spots, then 180 or 220 before primer.
Wet or dry
Dry.
Success check
The OSB is cleaner, duller, and free of loose raised fibers before primer.
What to do
- Remove loose debris.
- Sand high rough spots with 120 grit.
- Refine with 180 or 220.
- Vacuum dust before primer.
Avoid: Do not try to make OSB look like smooth plywood with sanding alone.