1. Hack off
all old plaster to at least 300mm clear of all signs of damp or salt
and at least 1 metre above ground floor level - use a meter to test
the walls and remove plaster at least 300mm above the highest level
that readings can be obtained.
This is to ensure that the residual 'salts' do not climb above the new
plaster in the future.
the walls to remove all plaster residue, particularly around angle beads.
old plaster will have 'salt' in it and will cause damp spots to appear
any holes or poor joints with sand and cement (4:1) using washed, sharp
(means slightly gritty) plastering sand (sometimes called screeding
or rendering sand). Do not use fine, unwashed sands.
fine, unwashed sands often contain salt and the fine particles are too
numerous to find enough cement particles to bond together tightly -
these two problems make for a weak render, prone to the easy passage
Note - walls made with non-porous stone or brick, like granite or blue
brick, will require an adhesive to help stick the render coat to the
wall - use BondAcryl concentrate)
normal renders and plasters stick to the wall by suction - the surfaces
need to absorb some water. Without this suction the render or plaster
will not grab the wall and may become loose and hollow as it dries.
5. Damp the
walls lightly (to reduce 'suction', which can cause excessive drying
and cracking) and apply a thin coat (maximum 1/2 inch, 12.5mm) of render
consisting of 3 parts sand - dry, washed, sharp (means slightly gritty)
plastering sand (sometimes called screeding or rendering sand) with
1 part of fresh (free flowing - no lumps) Portland Cement.
thick coats are more likely to slump down the wall during application
and crack during drying out.
the render surface liberally all over with a nail board, trowel, metal
float or similar object.
without these scratches the natural drying shrinkage will cause cracking,
crazing and hollowness to develop - the next coat will probably pull
the first coat off as it dries.
7. Use only
Renderproof water proofer/plasticiser in the water that the render is
mixed with, at the rate of 1 part Renderproof to 40 parts of water.
Do not use fine, unwashed or wet sands.
Renderproof binds the sand to the cement and prevents liquid water passing
through. It also makes the mix stickier (plasticising) which help to
hold the render together. Wet sand weighs more than dry, so it will
make your mix weak. Fine sands produce a weak, powdery render, which
will not resist 'salts'. Do not allow the plasterer to add plaster (usually
'browning') or washing up liquid to the mix.
8. When the
render surface is firm enough (but not bone dry, or it will need re-wetting)
apply a second coat to exactly the same specification - if further coats
will be needed to reach the desired thickness don't forget to scratch
liberally. In hot weather spray the render surface with water to slow
the drying process.
excessive drying out increases the suction and can prevent one coat
sticking to another. Rapid drying always increases shrinkage, which
gives rise to cracking of the render or finish plaster.
still damp (or re-wet again) apply a skim coat of Universal or Board
Finish. Do not polish or add water.
this will produce a shiny, glazed finish which looks good, but is prone
to condensation, black mould growth and poor drying.
any decoration for at least one month and then only apply a thin coat
of breathable emulsion paint, not a heavy vinyl. Our Replastering Paint
is the best option because it breathes and allows the wall to dry out
naturally. Do not repaint or wallpaper for at least three months.
the paint or paper will fall off the wall due to the water vapour that
will be trapped underneath in the new render.
here to see our Replastering Products Shop
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