Plastering: read the wall first

Cue 1 Water flicked at a wall: vanishing instantly, then sitting and running.

AMARA 00:00 I assumed plastering was all about the trowel.

NADIA 00:03 New plasterers think that. Experienced ones think about the wall, because perfect technique on the wrong background still falls off.

AMARA 00:11 What am I looking at when I look at a wall?

NADIA 00:15 Three things. Suction, which is how fast it pulls water out of your plaster. Strength, because your plaster must not be stronger than what it is stuck to. And stability, because a dusty or moving surface cannot hold a rigid finish.

Cue 2 Background types with the matching preparation beside each.

AMARA 00:32 How do I test suction? That sounds like something you need to feel.

NADIA 00:37 Flick some water at it. If it vanishes instantly the suction is high and your plaster will go off before you can work it. If it sits there and runs down, suction is low and the plaster will slide about and take forever.

AMARA 00:54 That is genuinely simple.

NADIA 00:56 Most of the good tests are. Then you match the material. High suction, old brick or blockwork, wants wetting down or diluted PVA, and traditionally an undercoat before the finish.

Cue 3 PVA film on smooth concrete failing, beside a slurry primer with aggregate keying.

AMARA 01:08 And a smooth wall? Concrete, or something painted?

NADIA 01:11 That is the one people get wrong. Low suction and smooth needs a bonding agent that gives you an actual key, so a slurry primer with aggregate in it.

AMARA 01:22 Not PVA?

NADIA 01:23 PVA on a smooth non porous surface gives you a shiny film and very little grip. People use it because it is what is in the van, and then the plaster sounds hollow six months later.

Cue 4 Gypsum on a lime background trapping moisture in a solid wall.

AMARA 01:38 What about a period house? Lime plaster on lath.

NADIA 01:41 Then lime, and never gypsum. And this is the same mistake as hard cement pointing on soft Victorian brick, exactly the same physics.

AMARA 01:50 Trapping moisture.

NADIA 01:51 Trapping moisture in a wall that was designed to breathe. Hard gypsum on a soft lime background in a solid walled house does real damage, and it is done constantly by people being helpful.

Cue 5 Four failure modes: blowing, crazing, a joint crack, delamination.

AMARA 02:05 Teach me to read failures. Plaster sounds hollow when I tap it.

NADIA 02:10 Blowing. It never bonded in the first place. Dust on the wall, a smooth surface with no key, or a missing bonding agent.

AMARA 02:19 Fine cracks all over the surface like a map?

NADIA 02:22 Crazing. It dried too fast. Usually high suction that was not controlled, or somebody had a heater or a draught on fresh work.

Cue 6 Cement burn developing over hours on a knee.

AMARA 02:31 A single straight crack?

NADIA 02:33 Look at where it is. A straight line almost always follows a plasterboard joint, which means the joint was not scrimmed, or the board is moving.

AMARA 02:43 Last thing, and I know the answer already because you said it about brick.

NADIA 02:49 Silica. Yes. Sanding plaster and cutting backgrounds generates respirable crystalline silica, and it causes silicosis and lung cancer. On tool extraction and a mask rated for fine particulates, not a nuisance dust mask from a pound shop.

AMARA 03:04 And the cement burns.

NADIA 03:05 Same as bricklaying. Slow, often painless at first, which is why people kneel in it and end up in hospital with full thickness burns. Waterproof gloves, keep it out of your boots, and wash it off the moment it is on you.

Cue 1 Set plaster residue in a bucket accelerating the next mix.

AMARA 03:22 My first attempt went hard on me halfway across the wall.

NADIA 03:27 Then start with the bucket, because that is almost always where it began. Was it clean?

AMARA 03:33 It had a bit of set plaster in it from before.

NADIA 03:37 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 03:49 So clean bucket every time.

Cue 2 Plaster into water, beside water into plaster trapping dry pockets.

NADIA 03:51 Clean bucket, clean whisk, clean trowel, every single mix. It feels excessive and it is the difference between a wall and a disaster.

AMARA 04:00 How do I mix it?

NADIA 04:02 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 04:15 How much?

NADIA 04:16 Add until it just breaks the surface, leave it a moment, then whisk until completely smooth with no streaks. And then stop.

Cue 3 First coat laid bottom to top and flattened, not finished.

AMARA 04:24 Why stop?

NADIA 04:25 Over mixing accelerates the set. You are shortening your own working time, which is exactly what a beginner cannot afford.

AMARA 04:33 How much should I mix?

NADIA 04:35 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 04:50 Right. Getting it on the wall.

Cue 4 Second coat going on as the first picks up.

NADIA 04:53 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 05:04 Not finish it?

NADIA 05:05 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 05:17 When does the second coat go on?

NADIA 05:20 As the first is picking up. Firm, but not hard. Thinner than the first, and it is filling imperfections rather than building thickness.

Cue 5 The trowel window: too early tearing, correct closing, too late immovable.

AMARA 05:29 And then the trowelling. This is the bit I have no feel for.

NADIA 05:34 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 05:47 How do I know?

NADIA 05:49 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 06:01 And the shiny finish?

Cue 6 New trowel corners rounded with a stone.

NADIA 06:02 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 06:12 Can I keep going over it?

NADIA 06:14 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 06:28 I get tram lines. Ridges.

NADIA 06:30 Trowel edge digging in. Either your angle is too steep, or the corners of your trowel are sharp.

AMARA 06:38 It is a new trowel.

NADIA 06:40 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.