Filling knife for wall cracks: fill or call someone
Updated: 21 October 2026
A hairline above the door frame looks like a five-minute job until the plaster behind it is hollow, or the crack widens every winter. A filling knife for wall cracks can press compound into a stable, sound line. It cannot reattach plaster that has left the wall, and it cannot stop structural movement. Prices for 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.
A hairline above the door frame looks like a five-minute job until the plaster behind it is hollow, or the crack widens every winter. A filling knife for wall cracks can press compound into a stable, sound line.
A hairline above the door frame looks like a five-minute job until the plaster behind it is hollow, or the crack widens every winter. A filling knife for wall cracks can press compound into a stable, sound line. It cannot reattach plaster that has left the wall, and it cannot stop structural movement. Prices for a blade are on the filling knife comparison. 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.
Our top picks
| Badge | Product | Widths | Blade flex | Handle | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Axus Decor Axus Decor Pro-Fill | Title says 2 inch / 50 mm | Ultra-flexible, per the listing | Rubber | 8.0 | about £5.39 | View offer |
| Alternative |  ROLLINGDOG ROLLINGDOG set | 1, 2, 3 and 4 inch | Some reviews wanted more flex | Beech wood | 6.3 | about £25.99 | View offer |
 Harris Harris Seriously Good | 2, 3, 4 and 5 inch | Flexible stainless blades | Rubber (TPR) | 8.5 | about £5.04 | View offer |
On this page
Hairline, fine, or something else
Toupret’s UK hairline guidance treats a true hairline as typically under 1 mm wide. At that scale you often do not undercut the crack. You do remove loose dust. Run the knife or a shave hook lightly along the line. If the plaster crumbles or dusts heavily, stop and stabilise first. Toupret says wait at least 24 hours after a stabiliser before you fill.
Wider cracks need a tape measure. Decoration Touch and Surveyor Local use about 3 mm as a point where a buyer’s survey may flag concern, especially if the crack is diagonal, wider at the top, or paired with sticking doors or sloping floors. An engineer’s article puts concern often from about 5 mm when movement is progressive. Those are habits, not one British Standard.
Knock first. A dense sound suggests solid plaster. A drum-like hollow suggests blown plaster. Decoration Touch says hollow means cut back and replaster.
Press across the crack
For a hairline that passed the dust and knock tests, Toupret 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. Water on the blade is optional smoothing on their how-to. One pass is often enough on a true hairline. A wider line may need a second thin coat after the first dries.
Use a narrow blade, towards 2 inch or the smallest in a continental pack, for a fine line. Feather wider damage with 3 or 4 inch steel. Harris lists 2 through 5 inch. ZAVIA does not number widths. Axus is one handled blade whose title width you measure. The width only matters once the crack is fillable.
Deeper than the tub allows
Polycell’s ready-mixed page limits a single fill to about 10 mm, then layers with drying between coats. That is Polycell’s figure, not every tub. Ask A Builder’s powder-filler context mentions stages when one pass would exceed about 6 mm on that guide. Read the label on the filler you bought.
Dusty or crumbling plaster needs Toupret’s stabiliser and the 24-hour wait, not thicker filler pressed harder. Toupret interior filler documentation also carries climate limits: not below 8 °C or above 35 °C, not above 70 per cent relative humidity, not outdoors. Apply those only to the Toupret line they belong to. Filler that skins before it cures can crack again along the same line. A knife cannot fix movement in the building.
When you put the knife down
Call a plasterer, a surveyor or a structural engineer when the crack reopens after a proper fill, when the knock says hollow, when plaster is collapsing, or when diagonal widening pairs with the movement clues Surveyor Local lists. Stop for lead when paint is failing on pre-1980s fabric where children live. HSE and PaintSafe are the sources. Leave a chase around cables you cannot isolate.
A 3 mm survey habit and a 5 mm engineering note are not one official limit. When the line is stable and within the tub’s depth, press and dry on how to use a filling knife. Width and handle are on the filling knife buying guide.
Common questions
How wide is a hairline crack?
Toupret’s UK method treats a hairline as typically under 1 mm wide. Measure if you are unsure. At that width, clear loose dust rather than gouging the line. A crack widening towards the top, or one near 3 mm with movement clues, is no longer that case.
Do you open the crack before you fill it?
A true hairline often needs dust removal, not a deeper chase. If the plaster crumbles or dusts heavily, stop. Toupret’s route is a stabiliser and at least 24 hours before filler. More compound on a crumbling edge does not replace that wait.
How deep can one layer be?
Polycell ready-mixed cites about 10 mm before you layer and let each coat dry. That figure is theirs, not every tub’s. Ask A Builder’s powder-filler guide mentions stages above about 6 mm. Read the label on the filler you bought.
What does a hollow sound mean?
A drum-like knock usually means blown plaster that has left the background. Decoration Touch says hollow means cut back and replaster, not fill over the void. A dense sound suggests solid plaster you might fill. A knife cannot reattach detached skim.
When is 3 mm enough to call someone?
Surveyor Local and Decoration Touch use about 3 mm, plus pattern and movement clues, as a survey concern. Diagonal cracks, a gap wider at the top, sticking doors or sloping floors sit in that picture. An engineering note often starts nearer 5 mm. Neither figure is a British Standard.
Why might a filled crack come back?
The building may still be moving, the plaster may be hollow, or the filler went on too thick and skinned before it cured. A knife only places compound where the substrate is sound. If a proper fill reopens, call a plasterer, a surveyor or an engineer.
Related pages
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.
- Toolstation, filling and putty knives (retrieved 17 October 2026)
- ProDec Advance 2 inch filling knife (retrieved 17 October 2026)
- Harris Ultimate 2 inch filling knife (retrieved 17 October 2026)
- Polycell Multi Purpose Polyfilla ready mixed (retrieved 17 October 2026)
- Toupret, hairline cracks (retrieved 17 October 2026)
- HSE, lead in construction (retrieved 17 October 2026)
- HSE, sanding plasterboard joints (retrieved 17 October 2026)
- Consumer Rights Act 2015, section 11 (retrieved 17 October 2026)
- British Coatings Federation, PaintSafe (retrieved 17 October 2026)