Plaster is the skin of your walls. It protects masonry from weather, hides uneven surfaces and gives a base for paint. Done well, it lasts decades. Done badly, it cracks within months. Most plaster problems come from four things: the wrong mix, the wrong thickness, poor surface preparation and no curing.
Typical thickness and mix
| Where | Typical thickness | Common cement : sand mix |
|---|---|---|
| Internal walls | About 12 mm | 1 : 5 or 1 : 6 |
| External walls | About 18 to 20 mm, often in two coats | 1 : 4 to 1 : 5 |
| Ceilings | About 6 to 10 mm | 1 : 3 to 1 : 4 |
Your engineer's specification takes priority. Thicker is not better: plaster thicker than needed is more likely to crack and debond.
Step-by-step
- Rake the joints of brickwork lightly and remove loose mortar and dust.
- Wet the wall the day before and again before plastering, so it does not suck water out of the plaster.
- Fix mesh over joints between concrete columns or beams and masonry. This is the single best way to stop vertical cracks at those junctions.
- Apply a cement slurry or bonding coat on smooth concrete surfaces so plaster grips.
- Use dots and screeds to set the thickness and keep walls straight.
- Apply in coats for thick external work, scratching the first coat to key the second.
- Cure by keeping the plaster moist for about 7 days, starting the next day.
Why plaster cracks
- Shrinkage: too rich a mix, too much water, or drying in sun and wind.
- Junction movement: concrete and masonry move differently. Mesh fixes this.
- Dusty sand: too many fines in the sand increase shrinkage.
- No curing: the surface dries before it gains strength.
Gypsum plaster: the alternative for interiors
Many builders now use gypsum plaster on internal walls. It goes on in a single coat, gives a smooth finish ready for paint without a separate putty coat in many cases, and needs no water curing. It is not suitable for external walls or wet areas.
Frequently asked questions
How long before painting new plaster?
Cement plaster should be allowed to dry thoroughly, often several weeks, before painting with an alkali-resistant primer. Painting too early causes blistering.
How much cement for plaster?
For 12 mm internal plaster at 1:6, a rough figure is around one bag of cement for about 120 to 130 sq ft, depending on wall evenness and wastage.