Answer
Go back and add the missing middle grits. After 120, use 150 or 180, then 220, then 320 so the deeper scratches are removed gradually.
Why it happens
The jump from 120 to 320 is too large for many surfaces, so 320 cannot quickly remove the 120 grit scratch pattern.
Recommended grit
After 120 grit, refine with 150 or 180, then 220, then 320.
Wet or dry
Use dry sanding for normal refinement. Use wet sanding only when the surface is water-safe and the grit stage is appropriate.
Success check
The old 120 grit lines are gone and the 320 grit pattern is even across the surface.
What to do
- Stop using 320 if deep 120 grit scratches remain.
- Return to 150 or 180 grit depending on scratch depth.
- Sand until the 120 grit lines are replaced by the new scratch pattern.
- Move to 220 grit and repeat the same check.
- Move to 320 only after 220 grit scratches are even.
- Clean the surface between grit changes.
- Inspect under angled light before finishing.
Avoid: Do not keep sanding with 320 hoping it will quickly erase deep 120 grit scratches. It usually takes longer than using the missing grits.