Why Plaster Cracks, and What You're Actually Looking At

Almost every home in the UK has a crack somewhere. Old lath and plaster walls move with the seasons, modern skim coats shrink as they dry, and thermal movement opens up hairline fractures above door frames and along ceiling joints. Knowing what you're dealing with tells you whether a straightforward repair will hold.

As a rule of thumb, hairline cracks under about 2mm are usually cosmetic. They come from drying shrinkage, minor settlement or the natural flexing of a timber-framed house. Cracks that are wider, that run diagonally from the corners of windows and doors, or that reappear within weeks of filling are telling you something structural is going on. Those need a plasterer or a surveyor, not a tube of filler.

You should also check whether the plaster is hollow. Tap gently along the wall with your knuckles. A dull, drummy sound means the plaster has lost its grip on the backing and no amount of filling will fix it. For everything else, a careful repair will give you a surface that paints up invisible.

Gathering the Right Kit

The difference between a repair you can see and one you can't comes down to preparation and the products you choose. Nothing here is expensive, and most of it you'll use again.

• A filling knife or flexible scraper, plus a narrower one for raking out
• Flexible acrylic filler (often sold as decorator's caulk) for cracks that move
• A fine powder filler for deeper holes and rigid repairs
• Sanding block with 120 and 180 grit paper
• Dust brush, vacuum with a brush attachment, and a damp cloth
• Dust mask and eye protection
• A small tub of primer or thinned emulsion for the finished surface

Flexible filler matters more than most people realise. Rigid fillers crack again as soon as the wall moves by a fraction of a millimetre. Acrylic caulk stays slightly elastic and absorbs that movement, which is why it's the sensible choice for hairline cracks in walls and ceilings.

Preparing the Crack Properly

This stage takes the longest and matters the most. Rushing it is why so many DIY fills show through the paint months later.

Start by raking out the crack with the corner of your scraper or a narrow filling knife. You're removing loose plaster, dust and any crumbling edges. It feels wrong to make the crack bigger, but a slightly widened V-shape gives the filler somewhere to grip. Flaking paint along the edges should come off too, back to a sound edge.

Next, deal with the dust. Vacuum it out, brush the edges, then wipe with a barely damp cloth. Dust stops filler bonding, so this step is not optional. If the exposed plaster is very porous or crumbly, brush on a little watered-down PVA or a dedicated primer and let it dry. This seals the surface so the filler doesn't lose its moisture into the wall too quickly.

Filling Without Making a Mess

For hairline cracks, apply flexible acrylic filler straight from the tube or with a fine nozzle, pushing it into the crack rather than laying it on top. Then smooth it with a damp fingertip or a small tool. One thin pass is nearly always better than a thick one.

For deeper cracks and holes, build up in thin layers with powder filler, letting each layer dry before the next. Most powder fillers shrink slightly as they set, so leaving the final layer just a fraction proud of the surface gives you something to sand back. Press firmly with the knife and draw it across the crack at a slight angle, then scrape flush with a single clean stroke.

Don't overfill. Excess filler means more sanding, and sanding too hard can drag the surrounding plaster out of level. If you're working on a ceiling, work in small sections and keep your knife clean between passes.

Sanding and Getting It Paint-Ready

Wait until the filler is genuinely dry. Acrylic caulk skins over quickly but stays soft underneath, and powder filler can look dry while still being damp at the base. Cool, humid rooms take considerably longer, so add a few hours to whatever the tub says.

Once dry, sand gently with 120 grit on a block, using light pressure and small circular movements. A block keeps the surface flat; your fingers will create dips. Feather the edges so the repair blends into the surrounding wall, then finish with 180 grit for a smooth surface. Wipe away all dust with a damp cloth.

Before painting, spot-prime the filled area. Filler absorbs paint differently from plaster, and unprimed patches show up as dull, flat spots — often called flashing — even under a good emulsion. A thinned mist coat of your topcoat colour is ideal, and two coats over the whole wall will leave the repair completely invisible.

Knowing When to Step Back

Fill, sand and paint will sort out the vast majority of household cracks. But if a crack is wider than 5mm, if it returns within a few weeks, if the plaster sounds hollow, or if you can see movement around a window or door opening, stop and get advice. Damp staining or a crack that widens noticeably over a season also points to a bigger issue.

Take your time, use flexible filler where movement is likely, and sand lightly rather than aggressively. Done patiently, a filled crack should disappear entirely under two coats of paint.