Repairing larger patches of plaster
preparing the wall - applying the first coat - applying the finishing coat
Repairs to small cracks and chips to plaster are covered on this other page "Repairing small damaged areas in plaster", repairs where large areas of plaster have become detached from the brick or blockwork are detailed below. The same techniques may be applied where the brick or blockwork has been repaired or replaced.
Preparing the wall
Start by removing all loose plaster, it will possibly extend further than just the visible area where it has come away from the brickwork. Use a light steel chisel and hammer towards the wall to remove any areas which sound hollow when tapped - don't use the chisel parallel with the wall at the point where the plaster meets the brickwork, this may detach perfectly sound plaster.
Once the area of loose plaster has been established, use a light chisel to remove mortar from the front face of the brickwork and to a depth of 5 to 7mm from between the exposed bricks - where the original mortar was lime based, very little force is needed, in fact hand scrapping with an old screwdriver may be all that is necessary.
Applying the first coat (the Browning or undercoat plaster)
Dampen the bricks and edges of the existing plaster using a light spray or just by flicking a brush damped with water towards the wall - the bricks/plaster needs to be damp but not running with water.
Mix up some Browning/undercoat in a clean plastic bucket, follow the manufacturer's instructions to get the required consistency.
Load some of the mixture onto a clean hawk. Hold the hawk against the wall below the area to be repaired (see right) and use a steel float to push a quantity of plaster onto the wall and then up the wall.
Start with the float at approximately 45° to the wall and as you move the float up the wall, reduce this to about 30°; pushing the plaster onto the brickwork and moving upwards at the same time.
When the plaster on the float has been spread, use an 'up and away' movement on the float to remove it from the surface.
Repeat applying the browning/undercoat mixture as described until the whole of the area of exposed brickwork has been covered.
Each Browning/undercoat layer should not be more than 10mm thick while being about 1mm below the surrounding plaster, when repairing existing plaster the thickness of this coat will be determined by the thickness of the existing plaster. Some old houses may have very thick plaster (upto 20mm is not uncommon), in which case, two coats of browning/undercoat should be applied to build-up the thickness.
Should a second coat of browning/undercoat be needed, roughen the first coat either by lightly scoring, in a criss-cross pattern' with the point of a trowel or by using a scratching float (basically, a wooden float with nails points sticking through the bottom). Leave the first coat to dry for a couple of hours, then wet the area and apply the second coat using the float and hawk in the same manner. It is a good idea to apply both coats in the same day to get good adhesion between them.
When the first coat has been built up as necessary, use a wooden straightedge across the repair to check that there is a gap for the finishing coat. Finish the Browning/undercoat coat by using a wooden float to achieve a flat but slightly rough finish.
Applying the finishing coat
The finishing coat needs more care, but is basically applied in the same manner.
Mix up some Finishing Plaster as per the manufacturer's instructions and load the hawk.
Push a small amount of plaster onto the wall and push it up and across the repair area using a steel float at about 30° to the wall as quickly as possible to give a thin coating.
Once the area has been covered and built up to the level of the surrounding plaster, polish the surface using the flat of a steel float. Start at the bottom left (if you are right handed) and sweep the float in an arc to the right. Slide the float off of the plaster and start again on the left but a little bit higher. Use the leading edge of the float in the direction of the arc and put a little pressure on the trailing edge of the float as you move it through the arc. If you are left handed, start at the bottom right and move the float in an arc to the left.
If the finishing coat starts to dry out while you are still working on it, dip the float into some water before continuing.
When the area has been polished, check for hollows and high points by looking along the surface; fill hollows with more mixed plaster, reduce high points by dipping the float in water and apply pressure as you sweep it over the area. Don't try to use a straightedge across the face as this will mark the surface and require a lot more effort to smooth out.