Sanding Marks Through the Finish

Drum stripes, edger swirls, witness marks. Where they come from, how to spot them in raking light, and how to fix without sanding back to bare timber.

What causes them

Sanding marks aren't a coating problem -- they're a preparation problem you only see after the topcoat goes on. Three main sources:

Grit skips

If you go from 40 to 80 without an 60 in between, the 80 grit is too fine to remove the 40-grit scratches and you end up with an even mat of fine scratches over a deeper coarse scratch pattern. The water-based topcoat fills both, and once it dries the deep scratches catch the light and read as straight lines following your sanding direction.

Bad belts or worn discs

A glazed-up or worn belt cuts unevenly, leaving harder-pressed bands wherever the belt overheated. Same goes for worn edger discs around the perimeter.

Edger pattern

The Bona Edge and similar 7" edgers leave a different scratch pattern from the belt because they cut in a circle. Where edger meets belt, the boundary often shows up later as picture framing -- the edge looks one shade and the field another.

How to spot them BEFORE you coat

Single most useful tool: a torch held flat against the floor. Walk every section under raking light at each grit. If you see lines, scratches or shadows, you go back. The cost of an extra 30 minutes with a torch is always less than the cost of recoating a whole room because witness marks showed up under the third coat.

The fix on a coated floor

If marks are showing through the topcoat, you have one good option and one bad option. Good: screen the floor with a maroon pad or 120-grit screen, recoat with the same finish, see if it hides. Bad: sand back to bare timber and start the grit sequence again from where the deepest scratch is. Don't try to spot-sand individual marks -- the recoat will show every patch line.

Stop it happening again

Run the full grit sequence: 36 -> 60 -> 80 -> 100 (or 36 -> 50 -> 80 -> 100 depending on the species). For tighter species like cypress and oak you can step finer. For ironbark and spotted gum you may need an extra grit at the start. Fresh belts and discs every job. Inspect under raking light at every transition. Don't let an apprentice run the edger before checking the field for marks.

Related: Sanding FAQs →