Answer
For MDF before painting, sand faces with 180 or 220. Seal edges, then sand them lightly with 220 or 320.
Why it happens
MDF faces and edges behave differently. Edges absorb more finish and can get fuzzy if sanded too aggressively.
Recommended grit
Use 180 or 220 for MDF faces. Use 220 or 320 lightly on sealed MDF edges.
Wet or dry
Dry.
Success check
The MDF is smooth, dry, and dust-free before primer or paint.
What to do
- Sand MDF faces with 180 or 220.
- Seal cut edges before final sanding.
- Lightly sand sealed edges with 220 or 320.
- Remove dust before primer.
Avoid: Do not wet sand raw MDF.