Answer
Use 180 or 220 on particle board edges, seal or prime, then scuff with 320 before paint.
Why it happens
Open particle board edges are porous and fragile, so they need gentle smoothing and sealing before paint.
Recommended grit
Use 180 or 220 for edges. After sealer or primer dries, scuff with 320.
Wet or dry
Dry.
Success check
The edge feels smoother and sealed before the paint coat.
What to do
- Sand the raw edge lightly with 180 or 220.
- Remove dust from the edge.
- Seal or prime the edge.
- After it dries, scuff lightly with 320 before paint.
Avoid: Do not sand particle board edges aggressively.