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How to fill cracks in walls

Updated: October 2026

How to fill cracks in walls is a sequence: clear the damage, press the paste in, stay inside that tub's layer limit, sand if it asks, paint the white fill, and put the lid back on. The [indoor wall filler comparison](/hub/fillers-adhesives-and-sprays/wall-filler-indoor) is where the packs and GBP prices sit. Which tub is the top pick, and which is best value, stays on the [indoor wall filler buying guide](/hub/fillers-adhesives-and-sprays/wall-filler-indoor/sub/wall-filler-indoor-buying-guide). This page owns the order of work. The limits were written up on 16 October 2026. This is not a lab test. A qualifying purchase can earn us a commission at no extra cost to you.

How to fill cracks in walls is a sequence: clear the damage, press the paste in, stay inside that tub's layer limit, sand if it asks, paint the white fill, and put the lid back on.

Our top picks

Polycell Polycell Multi-Purpose Polyfilla 330 g

Best overall

Polycell Polycell Multi-Purpose Polyfilla 330 g
9.3/10about £3.49
View offer
Palace Palace Fine Surface Filler 600 g

Best value

Palace Palace Fine Surface Filler 600 g
8.5/10about £1.25
View offer
Palace Palace Langlow filler 1 kg

Alternative

Palace Palace Langlow filler 1 kg
9.2/10about £6.99
View offer
On this page
  1. 1.Prepare the crack
  2. 2.Press, layer, then stop
  3. 3.Sand, paint, and the patch that still shows
  4. 4.Lid, tools, and when to stop

Prepare the crack

A smear over dust fails. Rake out crumbly plaster until what is left is firm, then brush the dust off. Prime a bare nail or screw head. Polycell's how-to asks for oil-based paint on nail heads. Filler will not stop rust bleeding through emulsion. Polycell's page says the surface and the crack should be dry. A housebuilder note says to dampen plaster first. Do not do both. Follow the tub in your hand. If that tub is Polycell ready mixed, keep the crack dry, and stir it before you load the knife.

Press, layer, then stop

Press a small amount into the crack, then strike it off along the line. A wet knife helps the last pass, but a soaking blade can pull paste back out. Leave the fill only just proud if you expect a little shrinkage. Stop at that product's own limit. Palace fine is layers no more than 3 mm. Sika's 600 g tub is 9 mm deep, then further layers. Palace's 1 kg tub is layers no thicker than 10 mm, with a skin in about 30 minutes and up to 24 hours at 20 °C. Polycell's multi-purpose page says to layer once you are above 10 mm. Let the layer firm before the next one.
Polycell Polycell Multi-Purpose Polyfilla 330 g

Testsieger

Polycell Polycell Multi-Purpose Polyfilla 330 g
9.3/10about £3.49
View offer

Sand, paint, and the patch that still shows

Sand when the product asks, once the fill is dry, and only the repair. No grit in these notes is universal. A thick Sika fill is harder to sand, which is why the layers should have stayed inside 9 mm. HSE construction dust is about silica on building sites, not a mask code for one screw hole. White filler still needs paint. Polycell's FAQ says to thin the first emulsion by about 20% if the fill flashes. If it still flashes, use a second full-strength coat.

Lid, tools, and when to stop

Ready-mixed paste dies in an open tub. Polycell's after-use notes say to replace the membrane and the lid. Palace reviews describe lids that split or fail to seal. Press the membrane flat, close the lid, and keep the tub off a frosty sill. Wash the knife in a bucket and let the solids settle. Polycell's multi-purpose page says not to flush filler or the wash water down the drain. Stop when plaster is loose over a wide area, a stain keeps returning, or the hole is beside wiring you cannot isolate.

Common questions

Do I rake the crack out first?
Yes, when the edges are crumbly. Loose plaster is why a neat smear opens under the first coat of paint. Take out what moves, brush the dust away, and leave sound material. A firm hairline still needs a slight open-up so fine filler can key. That is a slight open-up, not a wide V.
Should the crack be dry or damp?
Follow the tub, and do not merge the two guides. Polycell's how-to says the surface and the crack should be dry. A housebuilder note says to dampen plaster first. Doing both fights the product page. If the paste is Polycell ready mixed, keep it dry. A dampen line on another tub belongs to that tub alone.
How do I press the filler in?
Press the paste into the depth, then strike it off along the line. A skin across the surface leaves a hollow behind it. A slightly wet knife helps the last pass, as on Polycell's page, but a soaking blade can pull the fill out. Leave it only just proud if you expect a little shrinkage.
When do I stop and add another layer?
Stop at that product's limit. Palace fine is 3 mm layers. Sika is 9 mm, then another layer. Palace 1 kg is 10 mm layers, skinning in about 30 minutes and taking up to 24 hours at 20 °C. Polycell multi-purpose says to layer above 10 mm. Let the first layer firm. One thick pass past the limit is how the repair cracks.
Why does the patch show through the paint?
Filler is not paint. Reviews say a white fill still needed a coat, and one Polycell comment said to prime or the patch shows. Polycell's FAQ thins the first emulsion by about 20% if the fill flashes. A second coat usually evens the sheen. If a stain comes back through sound paint, stop.
How do I stop the tub going hard?
Replace the membrane and close the lid as soon as you finish. That is Polycell's after-use step. Palace lids in the reviews split or fail to seal, and the paste skins. Press the cover flat and keep the tub off a frosty sill. Wash the knife in a bucket, not the sink, and do not pour that water down the drain.

Related pages

Filler for hairline cracks

Fill a deep hole in plasterboard

Indoor wall filler

Best indoor wall filler: match the pack to the hole

Indoor wall filler buying guide: depth and movement

Sources

Public UK sources used for this comparison. Listing prices are the GBP figures captured for these amazon.co.uk ASINs and were not re-checked on 16 October 2026.

  1. Polycell Multi Purpose Polyfilla Ready Mixed (retrieved 15 October 2026)
  2. Polycell Fine Surface Polyfilla (retrieved 15 October 2026)
  3. Polycell Deep Gap Polyfilla (retrieved 15 October 2026)
  4. Polycell Advanced Polyfilla (retrieved 15 October 2026)
  5. Polycell Flexible Gap Polyfilla (retrieved 15 October 2026)
  6. Polycell — fill awkward or large gaps (retrieved 15 October 2026)
  7. HSE — Construction dust (retrieved 15 October 2026)
  8. HSE CIS36 — Construction dust (retrieved 15 October 2026)