Plastering is the first finishing stage, transforming rough brickwork into the smooth, even surfaces you will paint and live with. Done well it protects the walls and looks clean for years; done badly it cracks, bulges and shows every flaw through the paint. Here is what the stage involves.
What plaster does
Plaster is a thin coat of cement-sand mortar applied to walls and ceilings. Beyond giving a smooth surface for paint, it protects the masonry from weather, improves water resistance, and evens out the unavoidable unevenness of brickwork. External plaster in particular is your wall's first line of defence against rain.
Mixes and thicknesses
The mix and thickness are matched to the location:
- Internal walls: 12 mm thick, 1:4 to 1:6 cement:sand
- External walls: 15–20 mm, a richer 1:4 for weather resistance, often in two coats
- Ceilings: 6–8 mm, 1:3 to 1:4
A richer mix is stronger and more water-resistant but more prone to shrinkage cracks if overdone, so it is chosen to suit the job rather than simply maximised. External work usually gets a stronger mix and greater thickness because it faces the weather.
Estimating cement and sand
Plaster quantity starts from the wall area multiplied by the thickness to get a wet volume. Because mortar packs denser dry than the applied layer, that wet volume is multiplied by a 1.33 dry-volume factor — not the 1.54 used for concrete, because mortar has no coarse aggregate and bulks up less — before being split into cement and sand by the mix ratio. For a 10 m² wall at 12 mm in a 1:4 mix, that is roughly one bag of cement and about 0.13 m³ of sand. Using the wrong factor (1.54 instead of 1.33) is a common estimating error that leaves you over- or under-supplied.
Getting good results on site
Good plastering starts with preparation: the wall must be clean, and brickwork wetted so it does not draw water out of the plaster and weaken it. The surface is brought to a true plane using level guides, applied in even coats, and finished smooth. The most important step afterward is curing — keeping the plaster damp for several days so it gains strength rather than drying out and cracking. Rushed, uncured plaster is the usual cause of the hairline cracks that later telegraph through paint.
Common problems to avoid
Watch for plaster that is too thick in one coat (it slumps and cracks), too rich a mix (shrinkage cracks), inadequate curing (the biggest cause of cracking), and poor bonding to a dusty or dry surface (it sounds hollow when tapped and eventually falls off). A little supervision at this stage saves a lot of repair and repainting later.
Internal vs external sequence
External plaster is usually done first and is the more demanding of the two, since it must keep weather out; it is often applied in two coats — a rough base coat followed by a finishing coat — and may include waterproofing additives on exposed faces. Internal plaster follows, finished smoother because it will take paint directly. Ceilings are plastered thinnest of all. Plan the sequence so scaffolding and labour move logically around the building, and so that plaster has time to cure before the painting stage begins. Rushing from wet plaster straight to paint is one of the most common causes of finish failure, because trapped moisture pushes the paint off the wall within months.