Why will my filler not sand smooth?
Five distinct causes with five distinct symptoms — too soft, too hard, clogging, crumbling, or a repair that dishes as fast as you sand it.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 5 min
Short answer
If it clogs the paper it is not dry right through. If it crumbles it dried too fast or was mixed too wet. If it is glassy hard you have a two-part or a setting filler and need coarser paper. If the repair keeps going hollow you are sanding without a block. Work out which of those you have before buying more filler.
Sanding filler should be quick and it usually is. When it is not, the cause is nearly always something that happened before the abrasive touched it — the mix, the depth, the drying conditions or the substrate — and no amount of extra sanding fixes any of those. The symptom tells you which one it was.
Step by step
- Press a fingernail into an inconspicuous part of the repair.A hard, clean indent means it is set. Soft or greasy means it is not dry through. Powder under the nail means it never gained strength. That single test separates most of the causes.
- If the paper clogs, it is still wet inside.The surface can feel dry while a few millimetres down it is still soft, and sanding drags that wet material onto the paper. The only answer is more time, and much longer than the tub suggests for a deep fill in a cold room. Building up in coats prevents it.
- If it crumbles and powders, the water went too fast or the mix was wrong.A dry, dusty or freshly plastered wall pulls water out of the filler before it can set. So does a hot room or direct sun. And ready-mixed filler thinned with water, or powder filler mixed too wet, never develops strength. Seal a very absorbent surface first, and mix to the stated consistency.
- If it is glassy and will not cut, use coarser paper.Two-part fillers and setting powder fillers are genuinely hard. Start at 80 grit on a block to shape, then work through to 180 or 240. Trying to level a hard filler with fine paper is slow and burnishes it.
- If the repair keeps going hollow, use a block.Sanding a repair with paper wrapped round your fingers cuts the soft filler faster than the harder wall around it, so the middle dishes. A flat cork or rubber block spans the repair and cuts everything at the same rate.
- If you sand through to a ridge round the edge, feather it out.The mark you can still see after sanding flat is usually the edge of the filled area, not the middle. Sand well beyond the repair in a widening area so there is no step for light to catch.
- Fill the pinholes rather than sanding them out.Small holes in the surface are trapped air from application. Sanding deeper to remove them just makes the repair bigger. A skim of fine surface filler over the top and a light sand deals with them.
- Check with raking light, not by touch.Hold a torch almost flat against the wall. Every ridge and hollow shows as a shadow. This is how a decorator checks, and it finds problems that a hand will not.
Tips
- A wet sponge or a damp cloth levels ready-mixed filler beautifully while it is still soft, and removes most of the sanding entirely. Wipe the excess away before it hardens rather than sanding it later.
- Sanding filler produces very fine dust that gets everywhere and is unpleasant to breathe. A mask, and a vacuum rather than a brush.
- Do not skip the final wipe-down. Filler dust left on the wall stops paint adhering and shows as a dusty patch through the first coat.
- If a repair has taken three attempts, the problem is usually depth or suction rather than technique. Seal the wall and build up in coats.
- Fine surface filler exists precisely for the last skim over a repair. It sands to nothing and it is worth having a small tub.
If it doesn't work
The paper clogs and the filler smears
Cause: Not dry through — Fix: Stop and wait, and give a deep fill considerably longer than the tub states. Next time build up in coats thin enough to dry properly.
It powders away under the paper
Cause: The substrate sucked the water out, or the mix was too wet — Fix: Rake it out, seal the surface with a diluted primer or damp it down, and refill with a correctly mixed batch.
It will not cut at all
Cause: A two-part or setting filler, which is much harder than ordinary filler — Fix: Use 80 grit on a firm block to shape, then work up through the grits. Do most of the shaping at the cheese-hard stage next time, before it fully cures.
The repair is hollow in the middle after sanding
Cause: Sanding without a block — Fix: Refill proud and sand with a flat block that spans well beyond the repair.
It is still tacky and never hardens
Cause: Too little hardener in a two-part filler, or incomplete mixing — Fix: Scrape it all out — it will not improve — and mix a fresh batch to the stated ratio until the colour is completely uniform.
Questions people ask
How long should filler dry before sanding?
Until it is hard right through, which depends on depth, product and conditions. A shallow ready-mixed repair may be ready in an hour or two; a deep one in a cold room can need a day or more. Test with a fingernail rather than trusting the clock.
Why does my filler go powdery?
The surface pulled the water out before it could set, or it was mixed too wet. Seal a very absorbent wall with diluted primer first and mix to the stated consistency.
Can I sand two-part filler?
Yes, and it needs coarse paper on a block — 80 grit to shape and then finer. Shaping it at the cheese-hard stage a few minutes after it sets saves most of the work.
Why can I still see the repair after painting?
Usually either a ridge at the edge of the filled area or a difference in absorption. Feather the sanding well beyond the repair and prime the filled area before painting.