Plaster is used for covering brick and block walls, inside and out. But it is more likely that DIY users will be plastering interior walls to provide a smooth, solid surface for paint or wallpaper. Most interior plasters are made from ground gypsum (a natural rock) using a process that removes most of the moisture. There are three basic types of plaster: undercoat, finishing coat and one-coat plaster. However, each type has variations to suit particular surfaces and conditions.
Also pink or grey in colour, bonding plaster is used on surfaces that have low absorbency such as concrete, engineering bricks and surfaces treated with PVA solution. Bonding plaster is quick setting and should be applied within 15-30mins of mixing. Applying in layers 8mm (1/3in) thick will allow 10kg to cover 1.5m2. Once applied it will take about 1hr 30mins to 2hrs to set.
Used on absorbent wall surfaces such as bricks and blocks. It should be applied in layers roughly 10mm (3/8 in) thick. This pink or grey plaster is mixed with clean cold water and sets quickly in about 1hr 30mins to 2hrs, but this can vary according to room temperature. Browning plaster should be applied within 15 to 30mins of it being mixed. Applying the plaster at a thickness of 10mm (3/8in) 10kg will cover an area of 1.5m2
Plastering exterior walls is called rendering and involves applying a fairly thin layer of a cement, sand and lime mix. Although these types of exterior finishes are usually applied when houses are built, there may be times when repairs have to be made or the old plaster is removed and re-rendered. Render is applied in two coats: an undercoat called the float coat and a thinner, finishing coat.
If rendering an exterior wall, remember to carry out the work in mild weather as freezing temperatures will freeze the water in the render, causing it to crack. The typical render mix is:
- 6 parts plasterers’ sand (builders’ sand will cause the render to crack when it dries)
- 1 part cement
- 1 part hydrated lime
When mixed with water you should try to achieve a workable consistency: not too wet and not too dry. Render dries out quickly so don’t mix too much at one time, as it will be too stiff to trowel across the surface of the wall after about 20mins.
A finishing coat can be used for both browning and bonding undercoats. Once again it is pink or grey in colour and mixed with clean, cold water. It should be applied to the undercoat in a thin layer about 2mm thick, which means that 10kg will cover an area of 4.5m2.
Interior undercoat plaster
Plaster can be affected by wall surface absorbency. When plaster is applied to a wall the water is drawn from the plaster into the wall’s surface. If this happens too quickly then the plaster is likely to crack, so it is important to choose the correct undercoat for the wall you are plastering.
This is a popular alternative to the usual practice of applying an undercoat and finishing coat. One-coat plaster can be used on most indoor surfaces: bricks, blocks and plasterboard. Mixed with cold, clean water it will dry either white or pink depending on the brand. It can be applied in a layer of up to 50mm (2in) thick if filling in a cavity, but generally it is applied 10mm (3/8 in) thick. At this thickness 8kg will cover 0.7m2. One-coat plaster remains workable for between 30mins and 60mins depending on the room temperature.
A fast-setting compound used to repair damage to plaster walls and ceilings.
As its name suggests this finishing plaster should only be used on plasterboard. It must be applied to the ivory side of the board. Apply in a layer 5mm (1/5in) thick. Plasterboard finish sets in 1hr to 1hr 30mins and a 10kg bag will cover an area of 1.5m2.
To eliminate the mixing process plaster is available ready-mixed as an undercoat and finishing coat.
Ready-mixed undercoat plaster
Ready-mixed undercoat plaster will appear as a grey paste and is applied to the wall straight from the container. It is suitable for use on bricks, blocks, plasterboard and laths. It is slow setting, taking up to 24hrs to set, but this allows it to be workable for 4hrs after being applied. Generally applied in a layer 10mm (3/8in) thick, which means that 10kg will cover an area of about 0.75m2. One of the advantages is that if you are planning to wallpaper, the paper can be applied directly to the undercoat.
Ready-mixed finishing coat
A creamy/white paste that is applied directly from the container to plasterboard, ready-mixed undercoat or other plaster surfaces. Applying in a thin layer 3mm thick means that 10kg will cover 2m2. After being applied it will remain workable for up to 4hrs, but will take 24hrs to set.