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.
- Steel float
- String line
- Plasterers trowel
- Straight edge
- Spot (something to stand it on ‘a couple of crates’)
- Claw hammer
- Plumb level
- Splash brush
- Old saw
- Trimming knife
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.
The base or undercoat may need building up in two layers, to achieve the required thickness. The 2nd layer should be applied while the first is still damp but firm.
When mixing, add the plaster to the water in equal measures, using a clean bucket and clean water. When the plaster is a creamy constituency pour it out onto the spot board (a piece of ply about 1m square) at a comfortable working height. Use the trowel scoop up and load plaster onto the hawk.
To load the float
Use the float to scoop up and load plaster onto your hawk from the spot board. Now, hold the hawk, in the left hand if you’re right handed (or visa versa if you’re left handed), at a level between chest and shoulder. Set the blade of the float at right angles to the spot board and push a measure of plaster towards the edge furthest away from you. As you reach the hawk, tilt it slightly towards you and follow through with the float in a scooping motion. This movement is carried out in one fluid action. Practice this until you are satisfied with the progress. Preferably over the spot as this will catch any ‘droppings’.
When ready face the wall and work from the left hand ground, filling up a bay at a time. Push the plaster firmly against the wall and move upwards, keeping the blade of the trowel at an angle and flattening out at the end of the sweep.
When you have half filled the bay you are working on, run a straight edge up the grounds using it as a scraper to grade the thickness from ground to ground. As the straight edge fills up with plaster, bang the bottom of it on the spot and the residue of plaster will drop onto it saving waste and mess.Carry on until you have completed a full bay
When the first layer has gone stiff ,if a second coat is required, scratch the surface so that the second coat will key. An old wooden float with board nails in one edge is a good tool for this job (this is called a “devilling” float in the trade.)
When you have run the straight edge all the way up from top to bottom, fill in any dips left behind. When you have completed all the bays, let the plaster go off and then take off the grounds. Fill the troughs and wait for the browning to harden.
When the base coat has hardened, make a mix of finish.
Working from the floor level upwards, spread the plaster thinly (about 1/8″ thick), stopping just short of the ceiling then work downwards using the same action as described earlier. Tuck the float into the corner of the ceiling and work down to meet the upward sweep you have just spread.
When you have finished this coat you should be able to start ‘polishing off’.
When the wall has hardened, fully, go over it using a clean float and a wide paint brush, splashing on clean water, (use the water as required). This acts as a lubricant for the trowel, allowing it to pick up a fine film of plaster which fills any minute holes.
You will find that if you wash the tools at regular intervals this will not only save the hands from becoming sore it will also turn out a better end product.
Use the same technique for rendering as described in our plastering section. The materials required for rendering are well-washed plastering sand, gauged (mixed) with cement. A good idea is to use a plasticizer in the mortar, available from builders’ merchants. It acts as a lubricant between the fine grits, therefore helping ease of handling
For low suction backgrounds such as dense concrete blocks or dense brickwork, a sand cement ratio of 1:4 may be used for the first coat and 1:6 for the top coat.
For normal backgrounds such as general brickwork, normal concrete or clay blocks, a sand cement ratio of 1:6 may be used for the first coat and 1:8 for the top coat.
These ratios are for average conditions and a weaker mix (less cement) may be used where the anticipated exposure is more severe.
Patching lathe and plaster
To make good a hole in lath and plaster, cut back the damaged area to the nearest joist.
Screw a piece of 50mm by 25mm on either side of the hole.
Cut a piece of plasterboard the same size as the hole.
Using galvanized board nails nail the board over the damaged area.
Follow the guide for Skimming Plasterboard and apply a plaster finish.
Solid plaster corner forming
When putting on the grounds prior to plastering, at the corner, screw on a piece of 100 x 25mm (4 x 1inch) timber, with the outer edge in line with the grounds to the adjacent wall.
When applying the browning coat, plaster up to this timber form. Once the coat is stiff, unscrew this piece of timber and transfer it to the face of the wall you have just plastered. This will complete a right angle for you to use as a form for the corner. Plaster up to this form and, when the plaster has gone off, unscrew the piece of 100 x 25mm timber. When this has hardened you can “finish” plaster the walls. As you “finish” plaster you will find it easy to form a finished corner.