Module 1 of 2 · 40 minutes
Backgrounds and suction: why the wall decides the outcome
By the end of this module you will be able to
- 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
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 I assumed plastering was all about the trowel.
Nadia New plasterers think that. Experienced ones think about the wall, because perfect technique on the wrong background still falls off.
Amara What am I looking at when I look at a wall?
Nadia 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.
Amara How do I test suction? That sounds like something you need to feel.
Nadia 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 That is genuinely simple.
Nadia 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.
Amara And a smooth wall? Concrete, or something painted?
Nadia 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 Not PVA?
Nadia 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.
Amara What about a period house? Lime plaster on lath.
Nadia Then lime, and never gypsum. And this is the same mistake as hard cement pointing on soft Victorian brick, exactly the same physics.
Amara Trapping moisture.
Nadia 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.
Amara Teach me to read failures. Plaster sounds hollow when I tap it.
Nadia Blowing. It never bonded in the first place. Dust on the wall, a smooth surface with no key, or a missing bonding agent.
Amara Fine cracks all over the surface like a map?
Nadia Crazing. It dried too fast. Usually high suction that was not controlled, or somebody had a heater or a draught on fresh work.
Amara A single straight crack?
Nadia 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 Last thing, and I know the answer already because you said it about brick.
Nadia 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 And the cement burns.
Nadia 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.
The written material
Almost every plastering failure is a background failure
New plasterers obsess about the trowel. Experienced ones obsess about the wall, because a perfect finishing technique on a wrong background still falls off.
Three properties of the background decide everything. Suction, meaning how fast it draws water out of the plaster. Strength, because plaster must not be stronger than what it is stuck to. Stability, because a moving or dusty surface cannot hold a rigid finish.
Matching the material to the wall
High suction backgrounds such as old brick, blockwork and unpainted render need either wetting down or a diluted PVA coat, and traditionally a browning or bonding undercoat before finish plaster.
Low suction and smooth backgrounds such as concrete, painted walls, tiles and engineering brick need a bonding agent that provides a key. A slurry bonding primer with aggregate in it is the usual answer, because PVA alone on a smooth non porous surface gives you a shiny film and very little grip.
Plasterboard takes finish plaster directly, usually a multi finish, with the joints scrimmed first. Board with a grey face is designed for plastering; board with an ivory face is designed for direct decoration and is less absorbent.
Old lime plaster on lath needs lime, not gypsum. Putting a hard gypsum plaster onto a soft lime background in a solid walled period house traps moisture and produces exactly the same damage as hard cement pointing does to soft brick.
- High suction: wet down or dilute PVA, then browning or bonding undercoat
- Low suction or smooth: a slurry bonding primer with aggregate, not plain PVA
- Plasterboard: scrim the joints, then multi finish direct
- Lime background: lime plaster, never gypsum
- Never plaster onto anything dusty, loose, frozen or damp from a live leak
What each failure is telling you
Blowing, where the plaster sounds hollow when tapped, means it never bonded. Dust, a smooth non porous surface with no key, or a missing bonding agent.
Crazing, a fine map of cracks across the surface, means it dried too fast, usually high suction not controlled or heat and draught on fresh work.
A crack in a straight line along a board joint means the joint was not scrimmed, or the board is moving.
Delamination between coats means the first coat was too dry, too smooth or dusty when the second went on. Scratch the undercoat to give a key and do not let it dry out completely before the finish.
The dust and the burns
Mixing and especially sanding plaster generates dust. Cutting or grinding backgrounds generates respirable crystalline silica, which causes silicosis and lung cancer. Use on tool extraction and a proper mask rated for fine particulates, not a nuisance dust mask.
Cement based products, including some renders and screeds, cause severe alkaline burns that develop slowly and often painlessly at first. People kneel in it or let it sit inside a glove and end up with full thickness burns. Waterproof gloves, keep it out of boots, wash it off immediately.
Cement also causes allergic contact dermatitis, which once developed is usually permanent and ends careers.
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