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How to use a filling knife: press, level, then dry

Updated: 21 October 2026

You have a tub of ready-mixed filler and a flexible blade. The usual failures are smearing filler over the hole, leaving a ridge you sand for an hour, or rinsing the knife and tossing it wet into a drawer. How to use a filling knife, on this page, is the sequence UK manufacturers and retailers repeat: a small load, press, strike off, respect the depth on the tub, then wash and dry. Prices, if you still need a blade, are on the [filling knife comparison](/hub/painting-supplies/filling-knife). Peter F. did not run a lab test. A qualifying purchase through our links can earn us a commission at no extra cost to you.

You have a tub of ready-mixed filler and a flexible blade. The usual failures are smearing filler over the hole, leaving a ridge you sand for an hour, or rinsing the knife and tossing it wet into a drawer.

Our top picks

Axus Decor Axus Decor Pro-Fill

Best overall

Axus Decor Axus Decor Pro-Fill
8.0/10about £5.39
View offer
ROLLINGDOG ROLLINGDOG set

Alternative

ROLLINGDOG ROLLINGDOG set
6.3/10about £25.99
View offer
Harris Harris Seriously Good

Rank 3

Harris Harris Seriously Good
8.5/10about £5.04
View offer
On this page
  1. 1.Load a little and press it in
  2. 2.Across the crack, then level
  3. 3.Depth, then sand less
  4. 4.Wash it, then dry it

Load a little and press it in

Scoop a small amount onto the middle of the blade. A filling knife is not a trowel. Too much compound slides off and smears the surrounding paint. Toolstation’s guidance is a modest load, firm pressure into the defect, and a shallow angle so the steel flexes rather than digging into plaster. For a screw hole, centre the blade and push until the filler displaces trapped air. For a narrow crack, use the smallest width you own, about 2 inch or less, so the steel reaches the depth. Width choice is on the filling knife buying guide. Harris gives several widths. Axus asks you to confirm the 2 inch title is what arrived. Continental reviews report cuts, so keep fingers on the grip.

Across the crack, then level

Toupret’s hairline method draws the knife across the crack at a right angle, so material enters the gap. Polycell’s ready-mixed instructions say press in, do not overfill, and level with the surface. That how-to also allows dipping the knife in water to smooth. B&Q’s fill-holes guide says leave the repair slightly raised, then sand with 80 grit, and it names a dust mask and eye protection. Those level rules are not the same. Polycell warns against overfill. B&Q expects a slight proud line you sand back. Follow the tub you opened. After the across pass, use a shallow stroke along the wall to feather the patch into sound paint. The blade should bend slightly. If filler keeps pulling out of a deep chase, stop stacking height. That is a depth limit, not a cue for more pressure.
Axus Decor Axus Decor Pro-Fill

Testsieger

Axus Decor Axus Decor Pro-Fill
8.0/10about £5.39
View offer

Depth, then sand less

Polycell’s product page limits one pass to about 10 mm, then layers with drying between coats. Ask A Builder’s crack guide mentions stages above about 6 mm for a powder filler. Treat 10 mm as Polycell’s figure, not every tub’s. Levelling well is how you sand less. HSE’s limit for respirable gypsum dust is 4.0 mg/m³ over eight hours for employers, with on-tool extraction in trade work. Feathering beats aggressive dry sanding. B&Q still says wear a dust mask. Flaking paint on an older home is a lead question on the filling knife or stripping knife page.

Wash it, then dry it

Clean the knife before filler sets on the steel. Harris aftercare, on harrisbrushes.com and on the Seriously Good listing, is warm soapy water, then dry completely. Polycell’s pages say wash tools in warm water. Axus has no care line on the listing. Rinse the filler off and dry the blade anyway. Stainless on the packet is not permission to skip drying. Two of eleven UK reviews on ZAVIA say rust appeared after water and dispute the stainless claim. That is anecdotal, not a lab test, and it is why you towel-dry before the drawer closes. Continental edges stay sharp when wet. Point the blade away from your palm. If filler has hardened, soak briefly in warm water and use a cloth, not your thumbnail on the edge. Do not use a blow lamp on old paint while you hold this knife.

Common questions

What angle do you hold the knife at?
Hold it shallow to the wall when you feather a patch, so the blade bends. For a hairline, Toupret draws the knife across the crack nearer a right angle so filler enters the gap. Do not stiff-arm it like a scraper. If the steel will not flex, it is the wrong blade for filler.
Should the filler be flush or slightly proud?
Follow the tub you opened. Polycell says do not overfill and level with the surface. B&Q’s fill-holes guide says leave the repair slightly raised, then sand with 80 grit, and names a dust mask and eye protection. Those two ideas differ on purpose.
How deep can one pass be?
Polycell ready-mixed cites about 10 mm before you layer, with drying between coats. Ask A Builder’s powder-filler page mentions stages above about 6 mm. Treat 10 mm as Polycell’s figure. Read the label on the tub in your hand before one heavy pass.
Do you wet the blade while you fill?
Polycell’s how-to allows dipping the knife in water to smooth the repair. That is optional while the filler is workable. It does not replace drying the blade after you wash it. A damp edge for strike-off and a wet knife in the drawer are different steps.
How do you clean a filling knife?
Wash filler off in warm soapy water, the Harris aftercare line and the Polycell tool note, then dry the blade completely. Axus prints no care line. Rinse and dry anyway. If compound has hardened, soak briefly and use a cloth, not a thumbnail on the edge.
Why dry it if it says stainless?
A stainless line does not promise a wet blade stays bright in a drawer. Two of eleven UK reviews on ZAVIA say rust appeared after water. Harris aftercare is warm soapy water, then dry completely. Point a sharp edge away from your palm while you rinse.

Related pages

Filling knife for wall cracks

Best filling knife

Filling knife

Filling knife buying guide: width, flex and handle

Filling knife or stripping knife: flex decides the job

Sources

Public UK sources used for this comparison. Listing prices, stars and review counts are the amazon.co.uk figures saved for these four ASINs. The Harris price was also noted by Morris and Mica on 17 October 2026.

  1. Toolstation, filling and putty knives (retrieved 17 October 2026)
  2. ProDec Advance 2 inch filling knife (retrieved 17 October 2026)
  3. Harris Ultimate 2 inch filling knife (retrieved 17 October 2026)
  4. Polycell Multi Purpose Polyfilla ready mixed (retrieved 17 October 2026)
  5. Toupret, hairline cracks (retrieved 17 October 2026)
  6. HSE, lead in construction (retrieved 17 October 2026)
  7. HSE, sanding plasterboard joints (retrieved 17 October 2026)
  8. Consumer Rights Act 2015, section 11 (retrieved 17 October 2026)
  9. British Coatings Federation, PaintSafe (retrieved 17 October 2026)