Interior plaster
Plaster repair and skim coating.
Plaster is the first word in the name, and it is not a sideline here. Cracked walls, ceilings that have started to come loose, water damage from a leak upstairs, holes where something got opened up, and tired, over‑patched rooms brought back to one flat surface.
Plaster walls and ceilings
It helps to know what you actually have, because it changes the repair.
In an older house the plaster was pushed onto wood lath — thin strips nailed across the studs with a gap left between them. The plaster squeezed through those gaps and slumped over behind the wood, and those slumped‑over lumps are called keys. The keys are the only thing holding the wall on. In a house from roughly the 1940s through the 1960s the lath is more often gypsum board, with two or three coats of plaster over it. Newer than that and it is usually a thin veneer plaster over blueboard.
All three are harder, thicker and more brittle than modern drywall, and none of them are repaired the way drywall is repaired.
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Cracks
Fine cracks scattered over a wall are usually age and ordinary movement, and they are not going anywhere. The ones worth attention are the straight cracks over door and window heads, the long ones running out of a corner, and any crack you can get a fingernail into or where one lip sits higher than the other — that last one means the two sides are moving against each other. A crack gets raked out to sound material, backed if it needs backing, reinforced across the line, then built back in coats and blended well past the repair. Filling a plaster crack with joint compound and painting over it is what makes it come back through in a season.
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Water stains from a leak upstairs
The stain is the least of it. Plaster that has been soaked usually loses strength where it got wet — press it and it gives a little, or it comes away chalky. Paint over that and two things happen: the soft plaster keeps failing underneath the paint, and the stain bleeds back through in the first humid month. It gets taken back to plaster that is sound and dry, sealed so the stain cannot ghost through, then rebuilt and blended. The wall also has to be dry before anything closes it. If the leak is still live it gets fixed first, by whoever fixes leaks — sealing water inside a ceiling only moves the problem to next year.
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Sagging or loose plaster — broken keys
If you can push on a ceiling and feel it move, or tap a wall and hear a patch of it go hollow, the plaster has come away from the lath behind it. The keys have broken — from vibration, from a leak, or from a hundred years of a house shifting. Nothing you do to the face of that wall will fix it, because the face is not the problem. Where the plaster itself is still sound it can be drawn back tight to the lath and the framing with plaster washers and screws and then skimmed over, which keeps the original wall in place. Where too many keys are gone, or the wood lath itself has broken, that area comes out and gets rebuilt to the same thickness.
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Holes
A doorknob, a foot through a ceiling, an old vent or a pipe that came out. The first thing a hole needs is something solid behind it to work against, because a patch is only ever as good as its backing. Then it gets built up in coats to the real thickness of the plaster around it, which is usually a good deal thicker than a sheet of drywall. A flush drywall patch dropped into a plaster wall leaves a shallow dish, and in raking light you will find it from across the room for as long as you own the house.
Skim coating
Sometimes the right repair is not a repair at all. It is one thin coat over the whole surface instead of a hundred small patches you can still see.
A skim coat is a thin coat of finish plaster or setting compound troweled over an entire surface — corner to corner, not spot by spot. It is not a cosmetic wash. It is a new flat plane laid over the old one, usually in two or three thin passes, each worked in a different direction so the last one leaves the surface even.
It is the right answer when the surface is the problem and the wall behind it is not:
- a room that has been patched a dozen times over the years, where the patches are sound but you can pick out every one of them
- wallpaper that came off and took the paper face with it, leaving glue and torn spots behind
- a texture — knockdown, orange peel, an old swirl — that you no longer want
- a room where old plaster meets new board, and the join would otherwise show forever
- a ceiling, or a wall next to a window, where low light finds every ripple in it
Where it is the wrong answer: if the plaster underneath is loose, skimming it just puts a fresh face on a wall that is still coming down. Broken keys get secured first, or the new coat comes off with the old plaster.
Skim coating against replacing the wall
Both are legitimate, and we would rather tell you which one your room actually wants than sell you the one we happen to be standing next to.
| Skim coating makes sense when | Taking it out and re‑boarding makes sense when |
|---|---|
| The plaster is sound and tight to the lath, and it is the surface that is tired. | Most of the room is loose, or the wood lath behind it has broken up. |
| You want the original trim, casings and corners to stay exactly where they are. | The wall has to be opened anyway for wiring, plumbing, insulation or duct work. |
| It is an older house and the slightly hand‑made look of the walls is part of what you like about it. | The plaster has been wet a long time and has lost its strength across the whole area. |
| You would rather not empty the room and carry a wall out of it. | The room is being gutted for other reasons regardless. |
Which one it is gets answered by tapping and pushing on the wall, not by looking at it. That is a five‑minute job once somebody is standing in the room.
Putting the wall back after a leak or a repair
Plumbers, electricians and HVAC crews have to open walls and ceilings, and they should. Getting to a shower drain, a panel, a bad run of wire or a duct chase means cutting. Closing it back is a different trade from opening it.
What usually comes back is a rough rectangle of drywall screwed to whatever was handy — sitting proud of the plaster around it, or low, with a hard edge you could find with your eyes shut. It is not carelessness. It is that a plaster wall is thicker than the board being used to close it, and closing it properly is its own job:
- the backing gets set true first, so the patch has a flat plane to be built to instead of following whatever was screwed in behind it
- the patch is built up to the thickness of the plaster, not the thickness of the board
- the blend runs wide — far enough out from the repair that the change in plane never catches light at a low angle
- the texture is matched on the wall itself, not approximated at the truck
After a leak, let it dry before it closes. Framing and the back face of the surrounding plaster both need to be dry. A cavity that gets closed wet looks finished on the day and stains through by fall.
If a trade has already been in and left you an opening to deal with, a photo of it and a rough measurement is enough to start with.
Matching an existing plaster finish
Two things have to match, and most repairs that look wrong got only one of them.
Texture
Smooth troweled, sand float, knockdown, orange peel, a hand‑troweled swirl, skip trowel — each one is a different tool and, more than that, a different moment. The same material worked ten minutes earlier or ten minutes later leaves a different surface. That is why the match is worked out on your wall, on a piece of the actual surface, with the actual material, rather than picked off a sample board.
Sheen
This is the one that gets missed. An old plaster finish that was burnished with a steel trowel carries a faint polish that a modern compound does not have. Put a flat patch next to a burnished wall and you can still find the repair even when the texture is right, because light comes off the two surfaces differently.
And then paint
A repair that is genuinely invisible in bare plaster can still read as a rectangle once it is painted, because fresh paint on old paint shows whether or not the plaster underneath is right. Most repairs are worth painting out to the corners of the wall, and a ceiling is worth painting whole.
The hardest surfaces are ceilings and any wall a window rakes across. Low light finds everything. Those are the ones worth flattening wider than seems necessary at the time.
Show us the wall.
Send a photo and we will tell you what you are looking at — what system it is, what is likely causing it, and whether it needs doing now.