# Plastering: read the wall first

*Suction, backgrounds and bonding, because almost every plastering failure is a background failure.*

## Production summary

- Modules to record: 2
- Total script: 1027 words, about 7 minutes of finished audio
- Voices: Amara (host) and Nadia (practice educator)
- Level: Beginner to competent DIY

## Accreditation wording that must appear in the description

- **The CPD Certification Service** (planned): Application scheduled.

> Do not upgrade any of these words in a description or a thumbnail. Aligned is not accredited, and planned is not approved.


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## Backgrounds and suction: why the wall decides the outcome

**Runtime** about 3 minutes. **Words** 507. **Starts at** 00:00 in the full course recording.

### Learning outcomes to state on camera

- Assess a background for suction, strength and stability
- Choose the right plaster or bonding agent for that background
- Explain why the wrong choice causes blowing, cracking and delamination
- Control silica dust and cement burns

### Script


`[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.

### Sources for the on screen credit

- BS EN 13914-2, design and application of external rendering and internal plastering, British Standards Institution
- Control of silica dust in construction, Health and Safety Executive
- Cement and skin, Health and Safety Executive
- Lime plaster and traditional buildings, Historic England

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## Mixing, laying on and the timing of the trowel

**Runtime** about 3 minutes. **Words** 520. **Starts at** 03:22 in the full course recording.

### Learning outcomes to state on camera

- 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

### Script


`[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.

### Sources for the on screen credit

- BS EN 13914-2, internal plastering, British Standards Institution
- Gypsum plaster application guidance, Gypsum Products Development Association
- NVQ Level 2 Plastering, Construction Skills

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