Proper surface preparation plays an important part to achieve successful plastering. The Concrete Institute MD, Bryan Perrie, here provides advice on how to prepare a wide variety of surfaces before applying plaster:
In new plastering work, surface preparation should start with accurate setting out and construction of walls and soffits to provide a surface that can be plastered to the required lines and levels by applying a coat (or coats) of uniform thickness. Excessively thick plaster, or plaster of uneven thickness, should not be relied on to hide inaccurate work.
Where zones of the substrate surface deviate from the required plane or curved surface by more than about 10mm, first try to remove high areas by hacking or cutting. If not possible, apply undercoats to low areas in such a way that the final coat has uniform thickness.
For new work, masonry units that are strong enough to survive handling and transport should be strong enough to hold plaster. Similarly, in situ concrete should have ample strength.
Background surfaces should ideally be at least as rough as coarse sandpaper or rough sawn timber. Surface roughness can be achieved by:
* Using formwork with a rough surface, eg sawn timber, for substrate concrete
* Stripping formwork early and wire brushing concrete
* Abrasive blasting (eg sand blasting)
* Raking out mortar joints in masonry substrates to provide a key. A depth of about 10mm is normally adequate.
Roughness can also be achieved by applying a spatterdash layer. Spatterdash is a mixture of one part of cement (preferably CEMI or CEM IIA) to 1,5 parts of coarse sand with enough water for a sluggishly pourable consistence. A polymer emulsion should be substituted for part of the mixing water (usually a quarter to a third, but in accordance with the manufacturer’s instructions). The mixture is flicked on to the substrate as an initial coating to provide a key on dense or smooth substrates with poor suction. The spatterdash should cover the substrate surface completely and form a rough texture with nodules about 5mm high.
Spatterdash must not be allowed to dry out for at least three days and if a polymer emulsion is included in the mix, then curing should be in accordance with the manufacturer’s instructions. It should be tested for adhesion and strength by probing with a screwdriver or knife before plaster is applied.
Surfaces must be free of loose material such as dust, and films that can interfere with bonding, such as curing compounds and form-release oil. Substrate surfaces may be cleaned by:
* Water jetting
* Blowing with compressed air
* Vacuum cleaning
Solvents should not be used to remove films formed by curing compounds – such films must be removed by mechanical means.
Firstly, assess absorptiveness by throwing about a cupful of water against the surface. The surface will fall into one of three categories:
- No water is absorbed.
- Some water is absorbed – but most runs off.
III. Most of the water is absorbed.
Category I surfaces, which would include hard-burnt clay face bricks, glazed bricks and very dense high-strength concrete, should be prepared by applying a spatterdash coat that includes a polymer emulsion. Such surfaces must not be pre-wetted.
Category II surfaces should not require any treatment to control suction.
Category III surfaces should be wetted thoroughly for at least an hour and then allowed to become saturated surface-dry before the plaster is applied.
Preparation of various types of substrate:
Concrete is normally placed in situ but may be precast. Provide a rough surface by using rough-textured formwork, early stripping of formwork and wire brushing the concrete, hacking or abrasive blasting. If none of these is practicable, apply a spatterdash coat after ensuring that the surface is clean.
Ensure that no form-release oil is left on the surface to be plastered. Clean down by water jetting or vacuuming and remove curing compound, if any, by mechanical means. Conventional structural concrete should not require wetting to control suction but smooth off-shutter high-strength concrete surfaces will require the application of a spatterdash coat.
The texture of the masonry units should be sufficiently rough without further treatment. If not, apply a spatterdash coat and/or hack the surface. If the surface is dusty, clean by brushing, water jetting or vacuuming. It should not be necessary to control suction of the surface by prewetting, unless the masonry units are very absorbent.
Burnt clay stock brickwork:
The texture of the bricks will probably be sufficiently rough without requiring further treatment. If not, apply a spatterdash coat, hack the surface, or attach the new plaster mechanically with expanded metal lathing. If the surface is dusty, clean by brushing, water jetting or vacuuming.
Burnt clay stock bricks normally have a very high suction: this can be assessed by wetting the wall as outlined in earlier reference to absorption. If suction is high, pre-wet the wall and allow it to become saturated surface-dry before applying the plaster.
Burnt clay face-brickwork:
It is recommended that specialist advice be obtained for each specific case.
Poorly burnt soft clay brickwork:
This type of walling may be found in very old buildings, usually when restoration or repairs are being done. Care should be taken when removing the old plaster to prevent damaging the bricks. Protect the wall from rain or running water once the bricks are exposed. Rake out the joints about 10mm deep (the mortar is normally very soft). Brush down the wall to remove any loosely adhering material, then lightly dampen the wall and apply a spatterdash coat that incorporates a polymer emulsion to improve adhesion.