How to Fill Cracks in Interior Walls
Repair small cracks in plaster and drywall using filler, sandpaper, and paint for a smooth, invisible finish.
Quick answer
Widen the crack slightly, brush out dust, apply filler with a knife, let it dry, sand smooth, then prime and paint.
Small cracks appear in walls as houses settle and temperatures change. Hairline cracks are normal. You can fill them yourself in an afternoon without calling a decorator.
What you’ll need
- flexible filler or polyfilla
- filler knife
- sandpaper (120-grit)
- dust sheet
- decorators caulk (for larger cracks)
- primer
- paint and brush or roller
- vacuum or brush
Step by step
- 1
Prepare the area
Lay a dust sheet beneath the crack. Remove loose plaster or flaking paint with the edge of your filler knife. Vacuum or brush away dust so the filler can grip the surface properly.
- 2
Widen the crack slightly
For hairline cracks, run the corner of your knife along them to create a shallow V-shape. This gives the filler more surface area to stick to and prevents it from falling out.
- 3
Apply the filler
Load your filler knife with polyfilla. Press it firmly into the crack, scraping off excess. For cracks wider than 5 mm, use decorators caulk or a deep-gap filler first, then smooth over with standard filler.
- 4
Let it dry completely
Drying time varies from 30 minutes to 24 hours depending on the product and thickness. Check the manufacturer's label. Do not sand too early or the filler will crumble.
- 5
Sand smooth
Wrap 120-grit sandpaper around a cork block or use a sanding sponge. Rub gently in circles until the filler is flush with the wall and the edges are feathered into the surrounding surface.
- 6
Prime and paint
Brush or roll a coat of primer over the filled area. This stops the filler from absorbing the paint and flashing. Once dry, apply two coats of emulsion, feathering the edges into the surrounding wall.
If a crack is wider than 10 mm, runs diagonally across a wall, or is accompanied by bulging or bowing, it may indicate structural movement. Stop and consult a structural surveyor before carrying out cosmetic repairs.
Common mistakes
- Skipping the primer, which causes a shiny or patchy spot on the finished wall
- Overfilling and creating a bump instead of a flat surface flush with the plaster
- Painting before the filler is fully dry, causing cracks to reopen through the paint
- Using caulk instead of filler for deep cracks, which will shrink back and leave a groove
Frequently asked
Why does the crack keep coming back?
The wall may still be moving with seasonal temperature changes. Use a flexible filler designed for movement. If the crack reappears quickly, ask a builder to check for structural issues.
Can I fill a crack without sanding?
Only if the filler is perfectly level after application. Most jobs need light sanding for a professional, invisible finish.
What is the difference between filler and caulk?
Filler sets hard and is best for cracks and holes in flat plaster surfaces. Caulk stays flexible and is better for gaps between skirting boards and walls, or around door frames.
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