How do I skim a small patch of plaster?

Getting a patch flush with the wall around it, working with the plaster's setting stages, and knowing when filler is the better answer.

Difficulty: intermediate
Time: 2 hr
Read: 4 min

Short answer

Build the patch up with bonding coat to about 3mm below the finished surface, then apply two thin coats of finish plaster, flattening as it firms and polishing with a wet trowel just before it sets hard.

Plaster does not dry, it sets, and it passes through a series of stages in about ninety minutes. Working with those stages is what makes a patch flat; fighting them is what makes it a mess.

What you'll need

Bonding coat and finish plaster
A plastering trowel and a hawk
A bucket, a mixing paddle and clean water
PVA or a plaster bonding agent
A spray bottle
A straight edge

Step by step

Decide whether to plaster at all.Under about 300mm square, a good filler applied in coats is easier, cleaner and gives a comparable result. Plastering earns its place on larger areas.
Prepare the edges.Undercut the edges of the hole so the patch is keyed in, remove all loose material, and brush the dust out.
Control the suction.Old plaster and brick drink water. Diluted PVA or a bonding agent, allowed to become tacky, or a thorough dampening with water immediately before plastering.
Build up with bonding coat.For depth over about 5mm. Fill to roughly 3mm below the surrounding surface and scratch the surface to key the finish coat.
Let the bonding firm up.It should be hard enough to take the finish coat without moving. An hour or two, depending on the room.
Mix the finish plaster correctly.Plaster into water, never water into plaster, mixed to a smooth thick cream with no lumps. Clean bucket and clean tools — a trace of set plaster makes the new mix go off early.
Apply the first thin coat.Around 2mm, worked on firmly from the bottom upwards, covering the whole patch and lapping slightly onto the sound plaster.
Apply the second coat.About 1mm, once the first has firmed slightly, bringing the patch level with the surrounding wall.
Rule it off with a straight edge.Across the patch, bearing on the sound plaster either side. This is what makes the patch flush rather than proud or hollow.
Work the stages.Trowel it as it firms to flatten the surface, then again as it starts to lose its shine. Each pass tightens the surface.
Polish with water at the right moment.A light mist from a spray bottle and a final firm trowel just before it sets hard. Too early and you drag it; too late and you burnish streaks into it.
Do not overwork it.The most common mistake is trowelling continuously. Wait between passes, and stop once it is flat.
Let it dry before decorating.The pink colour goes uniformly pale as it dries — usually several days for a patch. Then mist coat before any topcoat.

Tips

Mix a small amount. A patch needs far less than you think, and finish plaster has a working life of around ninety minutes from mixing whatever you do.
Clean the tools and the bucket completely between mixes. Set plaster residue accelerates the next mix dramatically and is the usual reason a batch goes off in twenty minutes.
A straight edge ruled across the patch onto sound plaster either side is the difference between a flush patch and one you can feel through the paint forever.

Common mistakes

Applying finish plaster in one thick coat — Finish plaster is designed for about 2mm and then 1mm. Applied thick it slumps, cracks as it sets, and cannot be trowelled flat because the surface firms while the body is still soft.
Not controlling suction on old plaster or brick — A dry, absorbent background pulls the water out of the plaster before it can set properly. The result is a patch that goes off in minutes, cannot be trowelled, and ends up weak and dusty.

If it doesn't work

Patch sits proud of the wall

Cause: Built up too high — Fix: Once set, sand it back with coarse abrasive, or cut it out and redo.

Patch hollow

Cause: Not ruled off against the surrounding plaster — Fix: Apply a further thin finish coat, ruled off with a straight edge.

Plaster went off in the bucket

Cause: Dirty tools or contaminated water — Fix: Clean everything thoroughly and mix fresh; discard the batch.

Surface dusty and weak

Cause: Suction not controlled — Fix: Seal, and replaster; a weak surface will not take paint.

Trowel marks and streaks

Cause: Polished too late or overworked — Fix: Fill with fine filler and sand once dry, or accept and skim again.

Questions people ask

When should I use filler instead?

For anything up to about a foot square, a fine surface filler in two or three coats gives a comparable result with far less risk, mess and skill required. Plaster is worth it on larger areas or where a whole wall is being made good.

How long before I can paint it?

Until the pink colour has gone uniformly pale, which is usually three to seven days for a patch. Then a mist coat before any topcoat, because fresh plaster is extremely absorbent.