Rebecca Clarke
Why Cracks Appear in Plaster
Almost every older UK home has them: fine hairline cracks running up a wall, a spider's web in the corner of a ceiling, or a neat line tracing the edge of a plasterboard joint. They are rarely a sign that your house is falling down. Plaster moves with the building, and as timber dries out, heating cycles expand and contract walls, and the seasons do their work, the surface gives way along its weakest points.
What matters is that you deal with cracks before reaching for the paint tin. Emulsion has almost no thickness and certainly no gap-filling ability. Paint straight over a crack and the film will simply bridge the gap, then split again within weeks, leaving you with a visible line and a wasted coat. Filling is quick and inexpensive, and doing it properly is the difference between a wall that looks newly decorated and one that looks patched.
Identify the Crack Before You Fill
Not all cracks are equal, and the treatment depends on what you are looking at.
- Hairline and shrinkage cracks — fine, random, often across the whole wall. These are cosmetic and straightforward to fill.
- Tapered or joint cracks — straight lines along plasterboard joints or where an extension meets the original building. Often a sign of slight movement; a flexible filler or a reinforcing tape and skim may be needed.
- Deep or widening cracks — if a gap is wider than about 5mm, continues to grow, or runs diagonally from a corner of a doorway or window, stop and get it looked at by a surveyor or structural engineer. Filling it will only hide a problem.
- Crumbling, hollow or blown plaster — if tapping the area gives a hollow sound, the plaster has lost its key. That section needs to come off and be re-rendered, not filled.
Sort the cause first. Filling fresh is a waste of an afternoon if the crack reopens within a month.
Choosing the Right Filler
Your local builder's merchant will stock several types, and they are not interchangeable.
- Ready-mixed lightweight filler — the everyday choice for hairline cracks and small holes. It dries quickly, sands easily and does not shrink much. Ideal for interior walls.
- Powder-based filler — mixed with water on site. Stronger, sets harder and is better for deeper or wider cracks and for skimming over larger areas.
- Flexible acrylic or decorator's caulk — for cracks that will keep moving, such as where skirting, coving or window frames meet the wall. It stays slightly elastic. Do not use it on a flat wall where it will be sanded, as it will drag rather than flatten.
- Fine surface filler — a very smooth, thin finishing filler for the final pass over a repaired area. It sands to a feather edge beautifully.
Cheap filler shrinks and flashes through paint. It is worth paying a little more for a trade-quality product, especially if you are working on a large wall that will catch raking light.
Filling the Crack Step by Step
Preparation counts for as much as the filler itself.
- Run a filling knife or the corner of a scraper along the crack to open it slightly and remove loose material and dust. A crack with undercut edges holds filler far better than a tight slit.
- Brush or vacuum out the dust, then dampen the crack very slightly with a mist of water or a diluted coat of primer so the filler bonds.
- Press the filler in firmly with a flexible filling knife, working across the crack rather than along it. Overfill slightly, then draw the knife back at a shallow angle to leave the surface just proud of the wall.
- For deep cracks, fill in two passes rather than one. A thick single application will shrink, crack and possibly slump.
- Leave it alone. Most lightweight fillers need at least an hour, powder fillers several hours, and a full 24 hours in a cold or damp room. Painting too early is the most common reason repairs show through.
Sanding and Priming the Repair
Once fully dry, the filler will have shrunk slightly and sit a fraction below the surrounding plaster in the centre of the repair. Sand it flush.
Use 120-grit abrasive on a sanding block for the bulk of the work, then finish with 180 to 240-grit to feather the edges so you cannot feel where the filler ends and the plaster begins. Run your fingertips across the repair with your eyes closed; your hands will find ridges that your eyes miss. Wipe down with a damp cloth or vacuum attachment to remove every trace of dust, because dust is what makes paint crawl and look gritty.
Bare filler is far more absorbent than the surrounding painted wall. Seal it with a coat of thinned emulsion, sometimes called a mist coat, or a dedicated plaster primer. This stops the repaired strip drinking the first coat of paint and leaving a dull patch that no amount of extra paint will even out.
Painting So the Repair Disappears
Patience here pays off. Spot-prime the repair, let it dry, then apply your topcoats across the whole wall or ceiling rather than just the patched area. Painting only the repair nearly always leaves a visible edge, because the sheen and texture rarely match, even with the same tin.
Use a medium-pile roller and keep a wet edge, working in continuous overlapping strokes. Two thin coats are far better than one heavy one, which will highlight every imperfection under raking light. On a large wall with several repairs, consider a coat of slightly thinned emulsion first to unify the surface, then your finish coats on top.
Allow proper drying time between coats as well, ideally overnight in a well-ventilated room. Rush it and you risk dragging the filler, lifting the edge of the repair and starting the whole job again.
