Plaster can be applied to solid surfaces, such as bare brick, cement rendered brick, building blocks or concrete, and to surfaces to which a key for the plaster—such as metal lathing or wooden slats—is attached. On most solid surfaces two coats of plaster, known as the floating coat or undercoat and the setting or finishing coat, are applied. When lathing is fixed to the surface, an additional first rendering coat is necessary called a pricking up coat.
The main constituent of ready-mixed plasters is gypsum, calcium sulphate, which has been partly or wholly dried in a kiln. The extent to which the gypsum has been dried, and the addition of further constituents during manufacture, determine the type and grade of the plaster. However, as water is detrimental to gypsum plasters after they have set, they should not be used for external work.
Gypsum plasters, incidentally, should not be confused with cement render—a surfacing material made from sand, cement and lime and generally used externally. Render looks and feels like bricklaying mortar, which has similar constituents.
Lightweight, pre-mixed gypsum plasters are the most commonly used nowadays, by both professionals and amateurs. They come in several types, each used for a specific purpose. Browning is a floating coat plaster used on semi-porous surfaces such as bricks, clinker blocks and concrete blocks. Bonding is a floating coat plaster used on less porous surfaces such as poured concrete, where getting good adhesion is difficult.
Finish plaster is used for the thin surface coat that is applied over the undercoat.
Special plasters are also available for stopping and skimming plasterboard.
If in doubt on which undercoat plaster to use, ask the advice of your builder’s merchant.
Always store plaster in a dry place. If any water comes into contact with plaster before it is used, the properties of the plaster will be altered. You should use plaster as soon as possible after buying, as the retarder—the constituent which governs the setting time—grows less effective with time. Plaster is usually date stamped on the bag and, whenever possible, you should use the plaster within six weeks of the date.
Tools for plastering
Most of the tools you need for plastering can be made yourself. They include: Spot board: This can be a piece of exterior grade ply about 1m2 and is used for holding the mixed plaster. A couple of coats of exterior grade polyurethane wood lacquer will help to preserve the wood. The board should be placed on a stand—a wooden crate or sturdy stool will do—at a convenient height from the floor. The board should overhang the stand slightly so that the hawk can be held under the edge when transferring the plaster on to it.
Hawk: A board about 300mm x 300 mm for carrying plaster from the spot board to the work area and for holding the plaster as you work. Professionals use aluminium hawks with moulded-on handles, but you can get by quite comfortably with a home-made one. Cut your square from an offcut of timber or plywood and screw on a handle about 200mm long cut from 50mm x 50mm timber with the edges rounded off. Laying-on trowel: Used for applying and spreading the plaster. It has a rectangular steel blade about 280mm x 120mm attached to a wooden handle. Some trowels have curved handles which are easier to grip. A trowel of good quality is important as it is hard to obtain a smooth finish with a worn or inferior blade.
Gauging trowel: Available in a variety of sizes and used for repairing areas too small to be worked with the laying-on trowel. It is also useful for mixing small quantities of plaster. Skimming float: Used for levelling the floating coat. Plastic skimming floats, light in weight and non-warping, are available. But you can make a serviceable float from a smooth, fiat, straight-grained piece of wood about 280mm x 120mm x 10mm, with a wooden handle.
Scratcher: To ensure the adhesion of the next coat of plaster, the surface of an undercoat is scratched over. A suitable scratcher can be made by driving a few nails into a piece of wood and then cutting off their heads with a pair of pincers or pliers.
In addition, you will need two buckets—one for mixing the plaster in and one for holding water—a distemper brush and straight-edged rules of various lengths depending on the size and nature of the job. Also required, for chipping off old plaster, are a hammer and a bolster.
Preparing the surface
Before you start, clear the room of furnishings as much as possible, as the plaster dust will fly everywhere and can scratch polished surfaces. Cover what you cannot remove with old dust sheets. Have ready a suitable receptacle for the old plaster.
If the wall behind the plaster is of new brickwork it will need only brushing down and damping with clean water before you start to apply the new plaster.
Concrete wall surfaces require special preparation as their smoothness provides a poor ‘key’ for plaster and their density gives low suction. Before you plaster, paint the concrete with a PVA adhesive such as Unibond, applied neat.
Mixing the floating coat
When mixing plaster of any type use only water that is fit for drinking. Any impurities in water may be detrimental to the properties of plaster.
Pre-mixed lightweight undercoat for plastering small areas should be mixed a third of a bucket at a time. This is sufficient to cover a patch of about 300mm x 300mm to a depth of about 10mm. If the area to be plastered is larger than this, it is better to mix further amounts later. Pour water into the bucket first, then add the plaster. If the plaster is put in first, it clogs when the water is added and sticks to the bottom of the bucket. Add the plaster, while stirring the mixture with a stick, until a stiff but workable mix is obtained.
Whenever you have finished mixing any plaster, pour it onto the spot board. Then clean the bucket out immediately, or any remaining traces of plaster will set and then be extremely difficult to chip off. Traces of old plaster in the bucket will also speed up the setting time of subsequently mixed coats.
Applying the floating coat
Mix the floating coat plaster and place it on the spot board. Hold the hawk beneath the overhang of the spot board—if you are right-handed hold the hawk in your left hand and vice versa. Use the laying-on trowel to scrape some plaster onto the hawk then trim away any excess. Tilt the hawk and ‘snatch up’ a small amount of plaster onto the trowel. Keep the trowel horizontal until the edge connects with the wall, then tilt the outer edge upwards until it is at an angle of about 30° to the wall.
Begin in the centre of the patch and work upwards, exerting slight pressure. Keep the laying-on trowel at an angle, with its upper edge clear of the wall, so that plaster is fed to the wall all the time.
If the patch is 10mm deep or less, fill it until the new plaster is level with the old surrounding plaster. If the patch is more than 10mm deep, do not attempt to fill it in with one coat as this results in the plaster shrinking back from the edges and cracking. Instead, fill the area to half its depth, then use the scratcher to key the plaster with criss-cross lines. Apply a second layer of plaster when the first layer is dry.
Now, take a straight-edged rule a little longer than the patch and, working from the bottom upwards, draw the rule from side to side over the plaster to make it flush with the edges. Fill in any hollows with more plaster and draw the rule over the surface again.
To make room for the finishing coat, the plaster in the floating coat must now be cut back to a depth of 2mm lower than the surrounding plaster. First, flatten and cut back the floating coat with the skimming float. Next, run the scratcher over the surface of the floating coat to provide a key for the finishing coat. Then, go over the plaster with the skimming float again to flatten the burrs left by the scratcher. The scratch marks should remain but their edges should not protrude too far.
Clean the surrounding wall area to remove any adhering plaster and leave the floating coat to set. Ready-mixed plasters take between 1£ and 3 hours to set.
Before mixing your finishing coat, clean all tools and the spot board.
Mixing the finishing coat
Lightweight finishing plasters are applied thinly so they can always be mixed in a bucket. Pour water into the bucket until it is about a quarter full. Slowly pour in the plaster until it appears above the water and stir with a stick. Once the plaster has settled, add more and keep stirring until the paste reaches the consistency of thick cream. Then pour it onto the spot board.
Applying the finishing coat
Lightweight finishing coat plaster dries very quickly. So until you are experienced, mix and apply only enough to cover a small area at a time. Scrape some plaster from the spot board to the hawk and lift a small amount with the laying-on trowel. Use firm pressure to apply the plaster, using upward strokes as much as possible.
When the finishing coat is level with the existing plaster at the edges, draw the straight-edge over it until it is flush, filling in any hollows. As the plaster begins to set, dampen it with the distemper brush to keep it workable while you trowel it smooth. Do not use too much water as this can kill the gypsum plaster in the surface and cause crazing. Wet the laying-on trowel and, keeping it as nearly flat as possible, run it over the surface in circular movements, finishing off with light upward strokes. If you do not achieve a smooth, flat surface at the first attempt, dampen the surface and try again.
If a patch extends around an external corner, nail a batten with a smooth, straight edge to one side of the corner so that its edge is level with the existing plaster on the other side. Plaster up to the corner so that the new plaster is flush with the edge of the batten.