Types of plaster
There are quite a few plasters but two main types - gypsum based and cement based. The latter are used mainly for outdoors, generally for rendering. Gypsum based plasters are used indoors only - damp will attack a gypsum based plaster and cause it to crumble.
Cement based plasters can be used indoors for areas that are susceptible to damp and areas that require a greater thickness of cover.
The most common gypsum plasters are browning, bonding, and metal lathing plaster. Modern plasters come already mixed with perlite, vermiculite and several other additives. These additives provide a greater degree of insulation and fire resistance. Other ingredients slow down the setting time and generally make the mixture more workable.
Today the only additive we need to add is good clean water. When plastering materials with differing absorption, it is a good idea to key the surfaces to be plastered with a PVA bonding sealer. This prevents the plaster 'going off' (setting) at different speeds.
Browning plaster (perlite) - Brickwork, Coke breeze, Clay tile partitions, Thermalite blocks, Concrete bricks.
Bonding plaster (vermiculite) - Concrete, Stonework, Cork slabs, Surfaces treated with PVA.
Metal lathing plaster (perlite with vermiculite & rust inhibitor) - Expanded metal lathing, wood wool slabs.
One Coat Plaster - Used for any surface as undercoat and finish.
When buying plaster, buy only what is required, as it has a poor shelf life. A rough guide is as follows:
10kg should cover about 1.5 sq m at a thickness of 10mm.
The same weight should cover about 5 sq m at 3mm thick.
Spot (something to stand it on 'a couple of crates')
Check out Ron The Builder's video
Preparing the site for plastering
To help keep the plaster at a uniform plane all the way through the job, it is wise to use timber grounds. At first, set them vertically at 3ft/900mm centres and move up to 4ft/1200mm centres as you gain experience. A ground is a piece of timber that has been cut to the same length as the floor to wall height and fixed to the wall. The edges are bevelled to allow the ground to be removed when the plaster has 'gone off'. The thickness of the wood governs how thick the plaster is.
You can buy metal screed bead to use instead of grounds. These are designed to be left in the plaster, whereas when the base coat has 'gone off', the timber grounds are removed and the grooves filled with plaster.
The first ground is set at one end of the wall using a plumb level and straight edge, packing out, or screwing back any deviations from plumb. Set another at the far end of the wall. Now, you should have two vertical plumb grounds - one at each end.
Remember to set the grounds narrow face to the wall for ease of removal later!
From these two grounds, run a string line along the top and along the bottom. Pack the string line off of the grounds with a couple of 25mm wood blocks top and bottom to give some working space.
Mark the positions of the intermediate grounds, along the top and bottom of the wall. Loosely fix the grounds top and bottom, then pack out or screw back using your 25mm wood blocks to measure from the string. Once the top and bottoms are fixed, use a straight edged piece of wood, spanning top to bottom, to screw back or pack out intermediate fixings to the wall, so that the face of the ground is even and flat.
At external corners, use metal angle beads. These are stuck to the walls using plaster. Mix a bucket of plaster and apply about 5 dabs at intervals along the corner of the brick/block work.
Using a straight edge, line through from the grounds to the edge of the angle bead, and push it onto the dabs of plaster until flush with the edge of the straight edge. When satisfied, you can tack in to a mortar seam using a plasterboard nail - but double check after to make sure that it hasn't moved.
Line through from both walls that form the external corner, and if the grounds are plumb so will be the angle beads.
Any gaps around wooden linings such as door and window frames should be filled with a stiff mortar mix.