Why has my floor sanding left ripples and dips?
Drum marks, chatter ripples and dished patches all come from the machine rather than the abrasive. What each pattern tells you, and the honest options once they are in the timber.
- Difficulty
- advanced
- Time
- 4 hr
- Read
- 3 min
- Safety
- caution
Short answer
Scooped hollows come from pausing or lowering the drum while stationary. Evenly spaced ripples across the run are chatter from a worn drum, a loose abrasive wrap or a damaged wheel. Both only appear when the first coat of finish reflects light. The only real cure is to sand the whole floor back from a coarser grit, past the deepest hollow.
The cruel part of this fault is the timing. Sanded bare timber scatters light and hides an uneven surface almost completely, so the floor looks perfect right up to the moment the first coat of finish goes on and starts reflecting. Then every hollow and ripple appears at once, and by then the room is finished and the machine has gone back.
Safety
Step by step
- Look along the floor, not down at it.Get your eye close to floor level and look across the boards towards a window or a low light. This shows the surface profile in a way that looking down never will, and it is how you should have checked before finishing.
- Identify the pattern.Isolated scoops, often at the start or end of a run or where you stopped to change hands, are drum marks. Regular ripples spaced evenly along the whole run are chatter. A broad shallow dish across an area is over-working one spot with an edger or a coarse grit.
- Work out how deep they are.Lay a straight edge across the marks and look at the light under it. Marks visible only under a finish are shallow and might be removed with a fine corrective sand. Marks you can feel with a hand are deep.
- Check what timber you have left.Look at a board end at a doorway or under a lifted threshold. If the surface is already close to the tongue, or the wear layer on an engineered board is nearly gone, another full sand is not available and you have to choose a different answer.
- If you are correcting, go back a grit and start again.You cannot flatten a hollow with a fine grit — it follows the surface rather than cutting it. Go back to a medium or coarse grit and take the whole floor down to below the deepest mark, then work up through the grits again.
- Fix the cause before the second attempt.Check the abrasive is tensioned and fitted correctly, check the drum and the wheels for flats or damage, and correct your technique: walk first, drum down second, never stop, ease up while moving.
- Consider a planetary machine for the correction.A multi-disc machine cuts slowly and evenly and cannot gouge. On marks that are shallow, several passes with one of these will flatten them where a drum machine would only add new ones.
- If the floor cannot take another sand, change the finish.A matt or dead-matt finish scatters light instead of reflecting it and disguises a surprising amount of unevenness. So does a wire-brushed or textured look, and so, entirely, does paint.
If it doesn't work
Regular ripples every few hundred millimetres along every run
Cause: Chatter — a loose or badly seated abrasive wrap, a worn or out-of-round drum, or a flat spot on a wheel — Fix: Report the machine to the hire shop. Re-sand from a coarser grit with a properly fitted abrasive on a sound machine.
Deep scoops at the ends of the room
Cause: The drum was lowered before walking, or the machine stopped before the drum came up — Fix: Sand the whole floor down past the depth of the scoops, and change the technique at the ends of every run.
A dark ring or band around the edges of the room
Cause: The edger was worked to a coarser final grit than the main floor, so the perimeter takes stain and finish differently — Fix: Re-edge to the same final grit as the middle, feathering into the main floor, and re-finish. On a stained floor, this usually means re-sanding and re-staining the whole room.
Cross-grain scratches showing under the finish
Cause: A diagonal or cross-board pass that was never removed by the later along-the-grain grits — Fix: Re-sand along the boards from the grit that made the scratches, and work up carefully.
Marks only visible after the finish went on and only in one light
Cause: Shallow surface unevenness that the finish is reflecting — Fix: If it only shows in raking light from one window, a matt finish coat and a rug often close the case at a fraction of the cost of re-sanding.