Repair a patch of blown plaster — HomeGraft

Cut out loose or hollow plaster back to sound material, then patch and finish it flush with new plaster.

BP-01Clear and protect the roomMove furniture away from the wall or ceiling you're working on. Lay dust sheets over the floor and anything you can't move. Cutting out old plaster is dusty and messy, so open a window and put on a dust mask and goggles before you start.

1 each · Dust sheet

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1 pack of 5 · FFP3 dust mask

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1 each · Safety goggles

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BP-02Work out what's behind the plasterTap the wall to feel for solid brick or block (dense, no give), plasterboard on studs (hollow between studs, solid knock on the stud line), dot-and-dab (mostly solid-sounding with hollow patches at the dab gaps), or lath and plaster (springy, slightly uneven feel). Run a stud/cable detector, or a magnet, along the wall tracking a line of screw or nail heads to find studs. If you need to confirm timber versus metal, drill a small pilot hole in an inconspicuous spot: timber dust means a timber stud, fine grey swarf means metal.

1 each · Stud and cable detector

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BP-03Check for cables, pipes and asbestos before cuttingCables and pipes can run almost anywhere, not just in the usual safe zones, and a detector can miss ones that are deep, in metal conduit, or behind metal-backed board. Find your stopcock now, usually under the kitchen sink, and check it turns freely, so you can shut the water off fast if you hit a pipe. If there's any doubt, isolate the relevant circuit at the consumer unit before drilling or chiselling near sockets or switches, and turn off the water before cutting into anything that could carry it.

RULES

1 each · Non-contact voltage tester

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KNOW THE RULES

If your home was built or last plastered before 2000, old plaster, scrim, or textured coatings may contain asbestos. Don't sand or chisel it dry if you're unsure — get it tested first. This falls under the Control of Asbestos Regulations 2012, which places a duty to manage asbestos risk.

BP-04Sound out the full extent of the blown areaTap the plaster in a grid with your knuckles or a scraper handle. A dull, hollow, drum-like sound means the plaster has lost its key and is loose; a solid knock means it's still bonded. Mark the boundary in pencil, working outward until you hit solid plaster all round — always cut back further than you think you need to, or the edge of your patch will blow again later.

1 each · Pencil

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BP-05Cut out the loose plaster back to sound materialScore along your pencil line with a sharp knife, then chip away the loose plaster with a bolster chisel and club hammer, working back to the line. On solid brick or block, chip right back to bare masonry and undercut the edges slightly so the new patch keys in. On plasterboard, dot-and-dab, or stud walls, stop the moment you reach the paper face of the board — you're only removing the skim, and tapping the board itself tells you whether it's still firmly fixed. On lath and plaster, remove plaster only as far as the wooden laths are still sound and firmly fixed; broken or crumbling laths are a bigger repair than a skim patch.

1 each · Bolster chisel

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1 each · Club hammer

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1 each · Sharp utility knife

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BP-06Re-fix loose boards or laths before you patchIf the plasterboard itself, not just the skim, has come away, screw it back before filling. Into a timber stud, use a wood screw driven into the centre of the stud. Into a metal stud, use a self-drilling drywall screw made for steel track — never a plain wood screw, which strips straight through 0.5mm steel and never holds. On dot-and-dab, if the board has come off its adhesive dabs, use a fixing long enough to reach the solid masonry behind, with the right plug for that masonry — a short plasterboard screw alone won't reach. Metal studs won't support anything heavy or anything pulling away from the wall, so a large or heavy repair area needs the fixing taken back to a timber noggin or the masonry behind, not just the stud. On lath and plaster, screw a support batten across sound laths either side, or fix repair mesh, to give the new patch something to key into.

1 pack of 200 · Drywall screws, 40mm

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1 pack of 100 · Self-drilling metal stud screws

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1 pack of 10 · Masonry fixing screws, 75mm, with plugs

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1 roll · Plasterer's repair mesh

BP-07Prime the exposed surface and edgesBrush out all dust and loose debris from the hole. Dampen bare brick or block with clean water, or brush diluted PVA bonding agent onto masonry, board edges, and the old plaster edges — this helps the new plaster grip and stops it drying out too fast. Let the PVA go tacky, a few minutes, before you plaster over it.

1 litre · PVA bonding agent

1 each · Decorator's brush

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BP-08Build up deep patches with a backing coatFor holes deeper than about 10mm, apply backing or bonding plaster in layers no thicker than 10mm each. Scratch the surface of each layer with the edge of your trowel once it starts to firm up, so the next layer can key into it. Let each layer go off, firm to the touch, before adding the next, and bring the backing coat to about 2mm below the surrounding wall surface.

1 bag · Bonding/browning plaster, 25kg bag

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1 each · Plasterer's trowel

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1 each · Plasterer's hawk

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BP-09Apply the finish coatMix finishing plaster to a smooth, lump-free consistency. Apply a thin first coat with the trowel, then a second, flatter coat once the first has firmed up, working from the edges inward so the patch blends into the surrounding surface. Lightly spray water and polish with the trowel to close the surface and remove trowel marks.

1 bag · Finishing plaster (multi-finish), 25kg bag

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1 each · Water spray bottle

BP-10Leave to dry, then check and rub downLet the patch dry fully — a small patch can take 2-3 days, longer in a cold or damp room. Don't rush it with a heater; plaster dries best slowly and can crack if forced. Once it's fully dry, with no cool, damp patches when you touch it, lightly rub any high spots with fine abrasive paper or a barely damp sponge.

1 sheet · Fine abrasive paper

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BP-11Seal with a mist coat before decoratingNew plaster is porous and will drink up ordinary paint unevenly. Seal it first with a diluted mist coat, about 70% matt emulsion to 30% water, and let it dry fully before applying your normal decorating coats.

1 litre · Matt emulsion paint

1 set · Paint roller and tray

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BP-12Clean up and dispose of wasteSweep and vacuum up dust, and wipe down nearby surfaces. Bag up rubble and old plaster for disposal. Rinse plastering tools in a bucket of water, not down the sink, since wet plaster sets hard and can block pipes.

1 each · Rubble/waste sack

1 each · Bucket

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PART OF A BIGGER PROJECT

Repair a cracked, damaged room before decorating · Retile a bathroom wall

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