[edit] 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.

[edit] 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.

[edit] 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 (fig. 2). Tilt the hawk and snatch up a small amount of plaster onto the trowel (fig. 3). 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 (fig. 4).

Fig 2

Fig 3

Fig 4

Begin in the center 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 (fig. 5).

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 (fig. 6). 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 (fig. 8).

Fig 5

Fig 6

Fig 8

Fig 10

Next, run the scratcher over the surface of the floating coat to provide a key for the finishing coat (fig. 10). 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 1and 3 hours to set.

Before mixing your finishing coat, clean all tools and the spot board.

[edit] 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.

[edit] 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 (fig. 11).

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 (fig. 12) 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 (fig. 13). If you do not achieve a smooth, flat surface at the first attempt, dampen the surface and try again.

Fig 11

Fig 12

Fig 13

Q My newly applied plaster keeps cracking, I think because the old, porous plaster is drawing the water out of it. Is there a remedy ?

A Yes. Mix one part of PVA adhesive with four parts of water. Paint it onto the exposed brickwork and thoroughly soak the edges of the existing plaster. You should hear a 'fizzing' noise as the plaster soaks up the mixture; if not, the mixture is too rich. Apart from forming a water barrier, this treatment helps stiffen the old plaster.