Gypsum plastering — from bare wall to paint-ready surface
Gypsum plaster compresses what two cement coats and a skim used to do: one interior pass that comes out flat, smooth and paint-ready. But gypsum forgives nothing on two points: mixing water, and pace — its set time waits for nobody.
Where it belongs and where it does not
Gypsum is dry interior only: rooms, halls, ceilings, corridors. It stays out of bathrooms, sink surrounds and anywhere splashed — those take cement render then waterproofing. On smooth concrete it will not hold without a bonding bridge to key the surface; on absorbent block and brick it needs primer to regulate suction, or the wall drinks the mix water and the face crumbles.
Rails: flatness is built before plastering
Fix metal rails (or cast gypsum screeds) vertically every 1.5–1.8 m — a touch under your straightedge's length — and set them to one plane with level and line. External corners take perforated corner beads that armour the arris and keep it straight for good. This hour of setting-out decides the flatness of the whole room.
Mixing: powder onto water, never the reverse
Clean cold water in a clean vessel — hardened residue from the last batch accelerates the set of the new one and steals a quarter of your working time, so washing bucket and mixer between batches is not a nicety. Broadcast powder onto water at the bag's ratio, let it soak two minutes, mix to a lump-free cream. No added water after the set begins, and no "reviving" a stiffening batch.
Application and cutting flat
Apply with the trowel from the bottom upward between two rails, 8–25 mm per coat (thicker builds go in two passes, the first combed). Cut the face with the aluminium straightedge riding both rails in a rising zigzag — cutting twice beats once. As it firms (the sheen goes matt), take a final fine cut.
Polishing: after initial set, mist the face lightly and work it with a sponge float in circles — this raises the gypsum "milk". Then close it with a wide steel trowel. The result is a glassy face that needs no skim. Where extra perfection is wanted, a finishing putty goes over at 1–3 mm.
Drying before paint
Gypsum dries from the inside out; the working rule is a day per millimetre in ventilated air — a week to a fortnight for usual thicknesses. Never paint a face that feels cold or shows dark patches: trapped moisture under paint peels it. Prime with acrylic primer before painting, or the plaster's high suction drinks the first coat whole.
The gypsum mistakes
- A dirty mixing vessel — early set and a batch dying in your hands.
- Gypsum in a bathroom or at the kitchen sink.
- No primer on absorbent block: the face powders.
- No bonding bridge on smooth concrete: the plaster leaves in one sheet.
- Painting plaster that is dry outside and wet inside.
- Extra water to revive a setting batch.
Frequently asked
How long between two gypsum coats?
The second goes on after the first has set but before it fully dries (usually same day), the first combed for key. If the first has dried out completely, prime before the second.
Hairline cracks after drying — do I redo it?
Superficial hairlines are normal with fast drying and disappear under finishing putty before paint. Deep cracks tracking the blockwork joints are a substrate problem (movement, or no mesh where two materials meet) and need opening and mesh reinforcement.
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Last updated: 2026-08-09