How do I plaster a patch that has been taken back to bare brick?
Filling a 15 to 25mm deep hole in a wall so it finishes flush and does not crack at the edges — the two-coat method, the timing, and why the finish coat is the easy part.
- Difficulty
- advanced
- Time
- 2 hr 30 min
- Read
- 5 min
Short answer
Build the depth with a backing coat — bonding on a smooth background, hardwall on brick — applied in layers no more than about 10mm at a time, scratched to key, and finished a couple of millimetres below the surrounding surface. Let it set, then skim over the patch and well onto the existing wall in two thin passes.
Do this first
A patch back to brick is a different job from filling a hole. The depth means you are building the wall out, and the two plasters that do it have different jobs: a backing coat for bulk and straightness, a finish coat for the surface. The reason patches look wrong is almost never the finish — it is that the backing was not brought out to the right level, so the skim is either sunk or standing proud of the wall around it.
What you'll need
- Bonding coat or hardwall backing plaster
- Multi-finish plaster
- A plasterer's trowel, hawk and bucket trowel
- Two clean buckets and a mixing paddle
- A straightedge or feather edge
- Water brush and a small sponge float
- Scratching comb or a nail in a batten (optional)
- PVA or a suitable bonding agent, if the background needs it (optional)
Step by step
- Prepare the background properly.Rake out loose mortar, brush off every trace of dust, and cut the edge of the old plaster square if it is not already. Dust is the single biggest cause of a patch letting go.
- Control the suction.Dry brickwork drinks the water out of plaster before it can set, which is what causes a patch to go powdery and drop off. Either damp the background down thoroughly with clean water, or use a bonding agent appropriate to the background — one or the other, judged by how fast a splash of water disappears.
- Choose the backing coat for the background.Hardwall on ordinary brick and block with normal suction. Bonding on smooth, low-suction or mixed backgrounds — concrete, engineering brick, a patch that includes old plaster and new. Bonding also suits patches where the depth varies.
- Mix plaster into water, never the reverse.Clean bucket, clean cold water, plaster added while stirring until it forms a thick, smooth, lump-free mix roughly the consistency of whipped cream. Any contamination from a previous mix drastically shortens the working time.
- Apply the backing coat in layers.Firm pressure to push it into the background. No more than about 10mm in one pass on a wall — deeper than that and it slumps and cracks. Let each layer firm up before the next.
- Rule it off with a straightedge and check the level.Work the straightedge across the patch, riding on the sound plaster either side. Fill hollows and rule again. This is the step that decides whether the finished patch is invisible.
- Bring the backing to about 2 to 3mm below the surrounding surface, then scratch it.A comb or a nail scratched across it in criss-cross lines gives the finish coat a key. Leaving the backing flush with the wall is the mistake that forces the skim to stand proud.
- Let the backing set, then skim in two passes.First pass thin and firm, working from the patch outwards onto the existing wall well past the joint. Let it firm up. Second pass thinner still, filling and flattening.
- Trowel it up as it goes off.Plaster passes through stages: wet, firming, going matt, and finally hard. Trowel with a little water at the matt stage to close the surface, then a final pass with a clean, almost dry trowel. Working it too early tears it and too late scratches it.
- Feather the edge far wider than feels necessary.The transition from new to old is a thickness change, and the eye sees it in raking light. Take the skim 150 to 300mm past the patch and thin it to nothing.
Tips
- Mix small. Backing plaster gives you something like 90 minutes and finish plaster 60 to 90 depending on conditions, and a bucket that goes off on the hawk is wasted plus a bucket to clean out.
- Clean everything the moment you finish each mix. Set plaster on a trowel ruins the next coat by dragging lines through it.
- If the patch includes an external corner, fit a corner bead bedded in backing coat before you fill, and rule to the bead. Trying to form a straight arris freehand is a professional-level skill.
- On a patch bigger than about a square metre, screw temporary timber battens either side as depth guides and rule between them.
Common mistakes
- Applying the backing coat flush with the existing wall — There is then nowhere for the finish coat to go, so the patch finishes 2 to 3mm proud and shows as a raised panel forever.
- Plastering onto dusty or dry brickwork — The bond fails or the plaster dries rather than sets, and the patch goes powdery and hollow within months.
- Adding water to plaster that has started to go off — It re-tempers into a weak, crumbly material that never gains strength. Once it starts stiffening in the bucket, that mix is finished.
- Trying to make it flat by putting more finish plaster on — Finish plaster is a surface, not a filler. Depth is the backing coat's job and it has already set.
If it doesn't work
The patch has dried lighter and powdery
Cause: The background sucked the water out before it set — Fix: It has to come off. Control the suction properly and redo it — this one does not recover.
Cracks around the perimeter of the patch
Cause: The join between new and old was not reinforced or was applied over a dusty edge — Fix: Rake the crack out, scrim tape across the join and re-skim across it, feathering well out.
A visible step at the edge of the patch
Cause: Backing not ruled level, or the skim stopped too abruptly — Fix: Sand or scrape the high side back, then re-skim across the whole patch and much further out.
Questions people ask
Can I use filler instead of plaster for a deep patch?
For a hole a few centimetres across, yes, built up in layers. For anything approaching the size of a hand at full plaster depth, filler is expensive, shrinks and is hard to keep flat. Backing plaster is cheaper and behaves better.
How long before I can paint it?
New plaster needs to be dry through, not just dry on the face — commonly a week or more for a patch and considerably longer for a whole wall, depending on depth, ventilation and time of year. The colour going uniformly pale is the practical test. Then it needs a mist coat before ordinary emulsion.
Do I need to PVA the brickwork?
Not necessarily, and often not. PVA is a suction control and bonding aid, not a universal primer, and there are backgrounds where damping down or a proper bonding agent is the better answer.