Few things take the shine off a freshly decorated room quite like a hairline crack running across the ceiling or a patch of blown plaster beside a window. The good news is that most minor cracks and small areas of plaster damage are well within the scope of a competent DIYer. With the right filler, a little patience and a sensible approach to preparation, you can achieve a smooth, paint-ready surface that lasts for years.

Why Minor Cracks and Plaster Damage Happen

Hairline cracks are usually a sign that something has moved very slightly, not that your house is falling down. Common causes include seasonal thermal movement, the natural drying out of new plaster, and the gradual settlement of older properties. Plaster can also blow—meaning it loses its grip on the wall behind—when moisture gets in or when a previous repair was rushed. In Victorian and Edwardian homes, lath-and-plaster walls are particularly prone to fine cracking along joints and around cornices. Modern plasterboard ceilings often crack along the taped seams if the boards move or the jointing compound shrinks.

Most of these issues are cosmetic. A crack under one millimetre wide, with no sign of damp or movement, is a straightforward filling job. The key is to repair it in a way that accommodates future movement rather than fighting it.

Assessing the Crack Before You Reach for the Filler

Before you open a tub of filler, spend a few minutes looking closely at the damage. Run your finger along the crack, shine a torch at an angle, and check whether the edges are crumbly or firm. Hairline cracks that are stable and dry can be filled directly. Larger cracks—say, wider than a two-pound coin—or those that run diagonally from the corner of a window or door may need a professional assessment.

• Width and depth: Cracks over 2mm wide, or deep enough to swallow a filling knife, deserve caution.
• Movement: Does the crack open and close with the seasons? That suggests ongoing movement.
• Location: Diagonal cracks near openings, or step-shaped cracks in brickwork, can indicate structural issues.
• Recurrence: If a crack has been filled and painted before and has returned within months, the cause has not been resolved.
• Damp or staining: Brown marks or a musty smell point to water ingress, which must be fixed first.

If you spot any of these warning signs, stop and seek advice from a chartered surveyor or structural engineer. Filling and decorating over a structural crack is a waste of time and money.

Tools and Materials for a Neat Repair

For hairline cracks and small plaster repairs, you do not need a van full of kit. A basic set will see you through most jobs:

• A flexible interior filler—often labelled as flexible or crack-resistant—for hairline movement cracks.
• A powder filler for deeper holes and blown plaster, mixed to a smooth, thick paste.
• Decorator's caulk for gaps where skirting boards, architraves or cornices meet the wall.
• A filling knife or flexible palette knife, plus a smaller knife for tight corners.
• A sanding block and fine abrasive paper, around P120 to P180 grit.
• A dust sheet, a damp sponge, and a primer or mist coat for sealing the repair.

Avoid using rigid, general-purpose filler on cracks that are likely to move. Flexible filler costs a little more but stays slightly elastic, which prevents the crack from reappearing as soon as the heating comes on.

Step-by-Step: Filling Hairline Cracks

Start by laying a dust sheet and cleaning the area. Run a knife along the crack to remove any loose plaster or old filler, then brush out the dust. If the crack is very fine, you can widen it slightly with the corner of your knife to give the filler something to key into. Dampen the surface with a sponge—this stops the filler drying too quickly and cracking again.

Apply the flexible filler with a flexible knife, pressing it firmly into the crack and drawing the blade across at a shallow angle. Overfill very slightly, then leave it to dry according to the manufacturer's instructions. Once dry, sand gently with fine paper until the repair is flush with the surrounding wall. Wipe away the dust with a damp cloth. For blown plaster, cut back the loose material to a firm edge, then build up the repair in thin layers of powder filler, allowing each layer to dry before adding the next. Rushing this stage is the most common reason repairs fail.

When Larger Cracks Need Professional Assessment

Some cracks are more than skin deep. If a crack is wider than a pencil, runs diagonally across a wall, or is accompanied by doors that stick, floors that slope, or gaps around window frames, it is time to call in a professional. These can be signs of subsidence, failed lintels, or ongoing movement that will not be solved by filler and paint. A structural engineer can assess the cause and recommend a proper repair, which might involve underpinning, crack stitching, or replacing damaged masonry. Once the structural work is complete and the wall has stabilised, you can then return to cosmetic repairs with confidence.

Finishing and Decorating After Repair

Filler and bare plaster are thirsty, so always seal the repair before painting. A thinned-down mist coat of emulsion—about one part paint to four parts water—soaks in and provides a stable base. Allow it to dry, then apply your topcoats as normal. If the repaired area is in a spot prone to movement, such as above a door or along a ceiling line, use a flexible paint or add a little decorator's caulk to the filler for extra elasticity. Sand lightly between coats for the best finish, and keep a small pot of touch-up paint handy for future knocks. With careful preparation and the right materials, your walls will look smooth and settled for years to come.