Module 2 of 2 · 45 minutes
Mixing, laying on and the timing of the trowel
By the end of this module you will be able to
- Mix finish plaster to a consistent, workable standard
- Lay on and flatten within the working time
- Time the trowel passes so the finish closes rather than tears
- Diagnose a poor finish from what it looks like
Work through it
1 interactive for this module, built on the WAJD Teach engine. Nothing moves until you ask it to, and every one has a written version if you would rather read it.
Amara My first attempt went hard on me halfway across the wall.
Nadia Then start with the bucket, because that is almost always where it began. Was it clean?
Amara It had a bit of set plaster in it from before.
Nadia There it is. Set plaster acts as an accelerator. Your mix went off in about half the time it should have, and no amount of skill recovers that.
Amara So clean bucket every time.
Nadia Clean bucket, clean whisk, clean trowel, every single mix. It feels excessive and it is the difference between a wall and a disaster.
Amara How do I mix it?
Nadia Plaster into water, never the other way round. Water into plaster traps dry pockets that never wet out, and they turn up as lumps under your trowel at the worst possible moment.
Amara How much?
Nadia Add until it just breaks the surface, leave it a moment, then whisk until completely smooth with no streaks. And then stop.
Amara Why stop?
Nadia Over mixing accelerates the set. You are shortening your own working time, which is exactly what a beginner cannot afford.
Amara How much should I mix?
Nadia Less than you think. Everybody mixes a full bucket on their first wall and ends up scraping half of it into a bin while the rest goes hard. Two small mixes are far easier than one big one.
Amara Right. Getting it on the wall.
Nadia Two coats. First one about two millimetres, working bottom to top, trowel at a shallow angle so it does not dig in. Cover the area, then flatten it.
Amara Not finish it?
Nadia Do not try to finish it now. That is the single most common beginner mistake: polishing the first coat while the rest of the wall is going off behind you.
Amara When does the second coat go on?
Nadia As the first is picking up. Firm, but not hard. Thinner than the first, and it is filling imperfections rather than building thickness.
Amara And then the trowelling. This is the bit I have no feel for.
Nadia It is the entire skill, and it is felt rather than measured. Too early and you drag and tear the surface. Too late and it has gone and you cannot move it.
Amara How do I know?
Nadia The plaster stops sticking to the trowel and starts to firm under it. That is your window. You will feel it change under your hand before you can see it.
Amara And the shiny finish?
Nadia The final pass. A little water flicked on, a clean trowel held almost flat, long strokes, closing the surface. Once, at the right moment.
Amara Can I keep going over it?
Nadia No, and this is where people ruin a wall they had got right. Over trowelling burnishes the surface and causes crazing. Stop when it is right, which requires you to accept that it is right.
Amara I get tram lines. Ridges.
Nadia Trowel edge digging in. Either your angle is too steep, or the corners of your trowel are sharp.
Amara It is a new trowel.
Nadia Then that is it. Round the corners slightly with a stone before you use a new trowel. Every plasterer does it and nobody ever tells an apprentice.
The written material
Mixing, which decides the next forty minutes
Plaster into water, never water into plaster. The other way round traps dry pockets that never wet out and appear as lumps halfway through laying on.
Add plaster until it just breaks the surface, then leave it a moment before mixing. Mix with a whisk on a slow drill until it is completely smooth with no streaks, then stop. Over mixing accelerates the set and shortens your working time, which is the opposite of what a beginner needs.
Consistency: thick enough to hold on a trowel held vertically for a moment, loose enough to spread without dragging. Slumping off means too wet, tearing means too dry.
Clean tools and a clean bucket, every mix. A trace of set plaster in the bucket acts as an accelerator and your next mix will go off in half the time, which is how beginners lose a wall.
Laying on, and the timing that everybody gets wrong
Finish plaster is applied in two coats. The first is laid on to about two millimetres, working bottom to top, with the trowel at a shallow angle so it does not dig. Cover the area, then flatten it: do not try to finish it now.
The second coat goes on as the first is picking up, meaning it has firmed but is not hard. Thinner than the first, and it fills the imperfections rather than adding thickness.
Then the trowel passes, and the timing is the entire skill. Too early and you drag the surface and tear it. Too late and it has gone off and you cannot move it at all. The window is felt rather than measured: the plaster stops sticking to the trowel and starts to firm under it.
The final pass is with a little water flicked onto the wall and a clean trowel held almost flat, in long strokes, closing the surface. That pass is what produces a polished finish, and it happens once, at the right moment, not repeatedly.
- First coat about 2mm, bottom to top, then flatten
- Second coat as the first picks up, thinner, filling not building
- Trowel passes as it firms: too early tears, too late is immovable
- Final pass with a wet trowel almost flat, once, in long strokes
- Stop when it is right. Over trowelling burnishes and causes crazing
Reading a bad finish
Ridges and tram lines: the trowel edge is digging in, usually because the angle is too steep or a corner of the trowel is worn. Round the corners of a new trowel slightly with a stone before using it.
Blisters or hollow patches appearing days later: suction was not controlled, or the background needed a bonding agent it did not get.
A dull, dusty surface: over trowelled and burnished, or the final pass was made far too late.
Fine crazing: dried too fast. Heat, draught or uncontrolled suction. Keep the heating off and the windows shut while it goes off, which is counterintuitive and correct.
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