Answer
Guide coat left in low spots means the surface is not level. Use 320 or 400 carefully, then 600 after leveling.
Why it happens
Visible guide coat usually means the surface is not level yet or the low spot needs more filling or primer.
Recommended grit
Continue leveling with 320 or 400, then refine with 600 after the surface is even.
Wet or dry
Dry.
Success check
Guide coat is gone evenly without sanding through surrounding high spots.
What to do
- Stop and inspect the low areas.
- Continue block sanding with 320 or 400 only if enough primer remains.
- Add more filler or primer if low spots remain.
- Finish with 600 after leveling.
Avoid: Do not keep sanding until primer is gone on high spots.