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
- This is to ensure that the residual 'salts' do not climb above the new plaster
in the future.
Brush 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 in time.
Wet the walls and apply two coats of Salt
Neutraliser, wetting again with fresh water between coats
- old plaster will have 'salt' in it and the Salt
Neutraliser will reduce the
risk of these 'salts' resurfacing and spoiling the decorations in the future
4. Repair 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 of 'salts'.
5. Special 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
- 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.
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)
- thick coats are more likely to slump down the wall during application and crack
during drying out.
Scratch 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.
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 concentrate - dilute 40:1 with water
here to see the Products, prices or to Order
- 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.
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.
Whilst still damp (or re-wet again) apply a skim coat of Multi Finish 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.
Delay any decoration for at least one month and then only apply a thin coat of
breathable emulsion paint (not a heavy vinyl) - like our Microporous
Replastering Paint. Do not repaint or wallpaper for at least three months.
Use a Dehumidifier if possible, to remove moisture from the walls, especially
in the Winter months.
- the paint or paper will fall off the wall due to the water vapour that will
be trapped underneath in the new render.
Neutraliser - pdf
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