Start by Working Out Why the Crack Is There

Before you reach for the filler, spend five minutes working out what you are dealing with. A crack that has been there for years and looks exactly the same today as it did last winter is almost always cosmetic — the plaster has simply shrunk, or the house has settled gently into itself. A crack that is getting longer, wider, or reappearing within months of being filled is telling you something else entirely.

Run your finger along it. Hairline cracks under a millimetre wide, the sort that crazes across a ceiling or forms a spider's web in a corner, are surface shrinkage. Cracks tracking diagonally from the corner of a window or door, or running horizontally just above a door frame, deserve more suspicion — those are the points where the structure carries the most load.

Tap the plaster either side of the crack with your knuckles. A dull, hollow rattle means the plaster has blown, losing its grip on the backing. That needs cutting back rather than filling.

The Kit You Actually Need

Nothing here is expensive, and most of it will live in your toolbox for years.

• A filling knife — 50mm for narrow cracks, 100mm for wider work
• Flexible filler, the sort that stays slightly rubbery, for hairline and movement cracks
• Powdered setting filler for deeper holes and blown patches
• Decorator's caulk for joints where walls meet ceilings, skirting and architraves
• A sanding block with 120-grit and 180-grit paper
• A small scraper, a dust sheet and a spray bottle of water
• Fine surface primer, or a mist coat of emulsion

Buy flexible filler in a small tube if you can. The big tubs dry out long before you finish them, and you will only ever need a little at a time.

Filling Hairline and Shrinkage Cracks

These are the easy ones, and the method is deliberately gentle. Do not try to force filler into a crack that is barely open — you will smear it across the surface and create more sanding work than you bargained for.

Run the corner of your filling knife along the crack to open it very slightly and knock off any loose flakes. Lightly mist the crack with water, which helps the filler key and stops it drying too fast.

Press the filler in with the knife held at a shallow angle, dragging it across the crack rather than pushing it in, then scrape back flush in one confident stroke. Leave it slightly proud rather than slightly hollow — you can always sand off excess, but adding more to a dip is fiddly work.

Give it the full drying time stated on the tube. In a cold, unheated room that could be twice what the label suggests. Then sand with 180-grit, feathering the edges so you cannot feel where the repair ends.

Tackling Wider Cracks, Blown Plaster and Joints

A crack you can slide a 10p coin into, or any area that sounded hollow when you tapped it, needs a more honest repair. Cut back the loose plaster with your scraper until you reach material that is firmly stuck, and undercut the edges slightly so the filler has something to grip. Brush out the dust, then seal the exposed surface with watered-down PVA or a bonding primer and let it dry.

For anything deeper than about 5mm, build the filler up in two or three thin layers rather than one thick one. Thick filler shrinks as it cures and will crack again within weeks — which is exactly how most DIY repairs fail. Let each layer dry before the next, and leave the final layer just proud of the wall. Sand back flat, working along the length of the repair rather than across it, and check with the flat of your hand. Your fingers will feel a 1mm ridge that your eyes will miss.

Cracks where a wall meets a ceiling, or where plasterboard sheets join, are usually caused by normal movement between materials that expand at different rates. Rigid filler will simply crack again here, so use flexible filler or caulk instead. Open the joint slightly, dust it out, run a bead of caulk and smooth it with a damp finger. At plasterboard joints it is often worth cutting back the crack and bedding in a strip of scrim tape before filling, which bridges the join and stops it opening up again.

Around skirting, architraves and window reveals, caulk is the right answer almost every time. It stays flexible through the seasonal movement of the timber and will not leave you redoing the job each spring.

When a Crack Means Something More Serious

Some cracks are not a decorating problem. Treat these as a prompt to call in a qualified structural engineer or surveyor rather than reaching for the filler:

• Cracks wider than about 5mm, or any crack you can see daylight through
• Diagonal cracks running from the corners of windows and doors
• Horizontal cracks in brickwork, or a stepped crack following the mortar joints
• Cracks that reappear within a few months of repair, or that widen noticeably
• Doors and windows that suddenly stick or will not latch, or floors that feel noticeably sloping

Subsidence and movement are usually covered by buildings insurance, but only if you report them promptly. A repaired crack can complicate a claim, so if you suspect anything structural, get it looked at before you do the cosmetic work.

Getting a Finish You Cannot See

The filler is only half the job. The finish is what makes the repair disappear.

Once sanded, wipe the wall with a damp cloth to remove every trace of dust — leftover dust is the single most common reason a patched area shows through the paint as a dull, blotchy ghost. Spot-prime the repair with a thinned coat of emulsion or a fine surface primer. This seals the filler so it does not pull moisture out of your topcoat at a different rate to the surrounding plaster.

Then paint the whole wall, not just the patch. Feathering new paint into old is almost impossible to do invisibly, particularly in raking light or on a sheen finish. Two full coats over the entire wall, and nobody will ever know you were there.