Plaster Gone Wrong: Blown Patches, Damp Spots, How to Fix

Diagnose before you fix

A "blown" patch is plaster that has separated from the backing — usually a board or brick — while staying superficially intact on the face. Tap the suspect area with the back of a knuckle or a coin. Solid plaster sounds dull and dead. Blown plaster rings hollow.

• Hollow tap, no cracks visible — separated from the backing, still hanging on. Fixable by re-bonding.
• Hollow tap with hairline cracking — partially failed. Often fixable, sometimes needs cutting back.
• Hollow tap with stain or visible damp — there is a moisture source you must find first. Repairing the plaster without fixing the cause buys you 8–12 weeks before it blows again.
• Crumbling at the edges, sandy texture — backing failure. Cut back to sound plaster and re-skim that area.

Find the cause if there is damp

Plaster blows for one of four reasons: a leak (above, behind, below), condensation (cold-bridge wall, no extraction in bathroom/kitchen), rising damp (failed DPC), or background suction (new plaster applied to dry brick without proper PVA prep). The first three need fixing the cause before patching. The fourth is a workmanship issue and can be re-skimmed cleanly.

Re-bond a small blown patch (under ~20cm across)

If the patch is small, the face is intact, and there is no underlying moisture issue:

1. Drill 3–5mm pilot holes into the centre of the blown area on a 50mm grid, just deep enough to reach the cavity behind the face plaster.
2. Inject a structural plaster bonding adhesive (Carlite Bond-It or similar — about £12 a cartridge) into each hole until it backs out.
3. Press the face firmly back to the wall, weight or prop if needed, and leave 24 hours.
4. Fill the pilot holes with finishing plaster or fine filler, sand flush, redecorate.

Cut back and patch (medium areas)

For anything bigger than 20cm or showing edge cracks, cut the blown area out with a Stanley knife and a small bolster. Take it back to sound plaster. Wet the backing brick or board, apply diluted PVA (1:4), let it go tacky, then patch with bonding coat to within 2mm of the surface, leave to set, finish with multi-finish skim.

Drying conditions matter

Fresh patches need to dry slowly and evenly. Force-drying with a fan heater on a wet patch causes cracking. A dehumidifier in the room with the door closed and gentle airflow drops humidity without shocking the patch. Allow 3–5 days for a small patch before painting, longer if the room is cold or the original backing was damp.

The mistakes that cost a re-do

• Skipping PVA — patches fall off backing within months
• Painting over still-wet plaster — paint blooms, peels, or shows ghost lines
• Filling a blown area with cheap powder filler — moves under temperature changes, cracks again
• Not addressing the leak — the wall finds the same weak spot every time