Answer
Sand each grit until the previous scratches are gone and the pattern is even. Do not use time alone.
Why it happens
Time is less important than whether the previous scratches or roughness are removed.
Recommended grit
Use each grit until the surface has an even scratch pattern, then move to the next finer grit.
Wet or dry
Dry or wet depending on surface and stage.
Success check
Only the current grit pattern remains before switching grits.
What to do
- Inspect the surface under good light.
- Sand until the old scratches are replaced by the current grit pattern.
- Clean the surface.
- Move to the next finer grit.
Avoid: Do not move finer just because a set amount of time passed.