How do I fix a corner bead that has cracked?

External corners take every knock in a house, so the metal bead under the plaster is what keeps the edge straight. When that fails the crack follows the bead and filler alone will not hold.

Safety

What you'll need

• Replacement corner bead — galvanised or plastic
• Angle grinder or tin snips, and a bolster
• Filler or bonding coat and finishing plaster
• Taping knife, sanding block, primer

Step by step

1. Work out whether it is a knock or rust.A clean crack along one edge is impact. Brown staining that returns through paint is the galvanising failing and the steel corroding — that always spreads and the bead has to come out.
2. Check for damp if it is rusting.Bead in a dry internal corner should not corrode. Rust usually means moisture is reaching it, and that is the actual problem to solve.
3. Expose the bead.Scrape and chip back the loose plaster along the crack until you can see the metal and where the damage stops. Work along the corner, not across it.
4. Cut out only the damaged length.Cut the bead cleanly top and bottom of the failure, generously past the damage. Replacing a metre is far quicker than replacing a full-height corner.
5. Cut the replacement to fit exactly.Butt it to the remaining bead with no gap and no overlap. A step at the joint telegraphs through every coat you put over it.
6. Fix it straight.Bead adhesive, galvanised nails or plasterboard screws, checking with a straight edge down the corner as you go. The bead sets the line — if it is not straight, nothing after it will be.
7. Check the nose sits at the right depth.The rounded edge should sit just proud of the surrounding plaster so it can be covered by a thin coat. Too proud and the corner is a lump; too deep and it will show through.
8. Fill in thin coats.Two or three passes, each allowed to set, working from the bead out onto the wall. One thick coat cracks and shrinks away from the metal.
9. Feather a long way out.300mm or more either side. A corner repair is visible because of the hump where it stops, not because of the corner itself.
10. Sand along the corner, not across it.Across it rounds the arris off and the corner ends up soft and wavy. Long strokes parallel to the edge.
11. Prime before painting.Filler and new plaster are more absorbent than the wall. Without primer the repair shows as a different sheen even under two topcoats.
12. Consider plastic bead in wet or knock-prone places.It cannot rust and it survives impact better. Worth using in bathrooms, utility rooms and hallways where trolleys and prams go past.

Tips

• Replace a section, not the whole corner. Cutting out a metre is a morning; a full corner is a day.
• Check the bead with a straight edge before plastering. Everything after this follows the metal.
• Sand along the corner. Sanding across it is what makes repaired corners look soft.

Common mistakes

• Filling over a cracked bead without cutting it out — The damaged metal keeps moving and the crack reappears in exactly the same line within months.
• Painting over returning rust stains — Corrosion continues underneath and bleeds through again. Rusting bead has to be removed, not covered.

If it doesn't work