What is a plaster coat?
A plaster coat is a layer of mortar applied to a wall or ceiling that turns rough masonry or concrete into a flat, closed and decoratable surface. Plasterwork can be a single coat, but in practice it is usually built up in two or three.
Each coat has its own job, and that is not tradition but building physics.
The build-up
- Spatterdash or stipple coat. A thin, coarse, thrown-on layer that evens out the suction of the background and provides key. This coat deliberately does not cover the background completely.
- Floating coat (base coat). The load-bearing layer, usually 10 to 20 mm thick. This is where flatness is created: filling out, ruling off and floating up. This coat provides the strength.
- Finishing coat. Thin (a few millimetres) and decisive for the appearance: trowelled smooth, floated, sponged or textured.
The golden rule: soft on hard
Each successive coat must be softer (weaker) than the one beneath it. A hard, cement-rich finishing coat over a soft lime background will inevitably crack and debond, because it cannot follow the background’s movement. The other way round it works.
The same rule governs the choice of binder. An old, vapour-open wall of soft brick and lime mortar wants lime or clay plaster, not a cement-based one. Seal such a wall with a vapour-tight coat and moisture builds up behind the plaster, bringing salt efflorescence and frost damage.
Practical points
- Thickness per pass. Applying cement-based plaster too thick in one go causes slumping and shrinkage cracking. Build up in passes and respect the stated maximum thickness per coat.
- Waiting time between coats. Each coat must have hardened sufficiently before the next goes on; with cement and lime binders that quickly means days per coat, not hours.
- Prepare the background. Dust free, sound and with even suction. Prime strongly absorbent backgrounds, give smooth ones a bonding bridge, or use a plaster base.
- Not in full sun or frost. Drying too fast means incomplete hydration and therefore a weak, dusting coat.
Related terms
- Plaster base
- Render
- Finishing plaster
- Lime mortar
- Clay plaster
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