BACKGROUNDS AND PREPARATIONS
General:
 Gypsum plasters should never be applied to areas that are frozen or are likely to become
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frozen before the plaster is fully set and dry.
Plaster can be used in frosty conditions as long as the plastered surfaces can be
adequately protected after plastering.
If plastering in hot / dry conditions protect the work to ensure that rapid loss of water is
avoided.
If water in the plaster is dried off too quickly this will affect the strength of the set
plaster.
Background Preparation :
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The background should be dry and protected from the weather.
The suitability of the background for plastering should be given the following
considerations; its strength, suction, bonding properties, any shrinkage or thermal
movement possibilities and water and salt content.
Backgrounds :
Brickwork / Blockwork
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The surface must be clean and dry.
Providing the suction is not too severe it can be controlled with water or treated with PVA
before plastering. If plastering onto high suction brickwork / blockwork, the use of Carlite
Tough Coat should be considered.
Low suction backgrounds such as concrete blocks, engineering bricks and some calcium
silicate bricks (facing bricks) give minimal absorption. Any joints should be raked thoroughly
to give a good mechanical key.
Dense aggregate concrete blocks should not require wetting before plastering but the plaster
needs to be applied with a good firm pressure to ensure contact with the background.
Aerated blocks (Thermalite) can give high suction. This suction can be controlled by pre
treatment with clean water or a PVA bonding agent.
BACKGROUNDS AND PREPARATIONS
Concrete
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The surface must be clean and dry.
Make sure there is no mould release agent or oils on the surface.
No fines concrete should not require wetting before plastering.
Concrete should be given time to mature before applying plaster.
Plaster should not be applied to a ‘green’ background.
Mature concrete needs wetting before plastering to displace the air present in the surface.
Apply clean water about 5-10 minutes before plastering to control the suction.
Pre cast concrete which is exceptionally smooth will need treating with a bonding agent such
as PVA.
Pre cast concrete are low suction and should be plastered with Carlite Bonding.
Carlite Bonding works by creating a vacuum against the surface of the concrete.
Sand / Cement undercoats
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Using the correct grade and type of sand and allowing sufficient drying time for drying
shrinkage of the sand/cement are essential to reduce the risks of any defects in the work.
Sand/cement needs to be allowed to dry thoroughly to allow for the shrinkage movement to
be completed before the finish is applied.
The suction in sand/cement undercoats can be controlled by sprinkling clean water on the
surface with a flat brush.
Overly sanded surfaces can lead to loss of adhesion of the finish coat and can result in a poor
finish.
Sand/ cement undercoats need to be keyed thoroughly with a moderately nailed devil float
to achieve a good key.
If the key of the devil float is too deep air is trapped in the key lines resulting in a blistered
finish.
BACKGROUNDS AND PREPARATION
Expanded Metal Lath ( EML)
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Plaster should only be applied to galvanised steel or epoxy coated stainless steel.
Before applying plaster make sure any cut edges, damaged metal lath, staples nail heads and
tying wires are bent inwards.
Apply a pricking up coat first to the EML and then apply a good scratch with a scarifier.
Before applying the floating coat the pricking up coat must be allowed to set but not dry.
Generally EML is used on low suction and poor backgrounds so Carlite Bonding is advised to
be used.
Rib lath is EML with a creased rib every 100mm across its width with the ribs running down
the full length of the sheet.
Use the rib placed against the background surface for strength and to ensure the material
achieves a good thickness.
Backgrounds using PVA bonding agent
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All PVA agents comply with BS 5270: part 1 : Bonding agents for use with Gypsum plasters
and cement.
Always use these bonding agents in accordance with the manufacturer’s instructions.
Generally if the thickness of your undercoat is over 8mm and is relying on PVA for adhesion
another method for adhesion should be used. The weight of the material will be too much
for the PVA adhesive.
Backgrounds such as ceramic tiles, glazed bricks or concrete made from limestone, granite
and certain lightweight aggregates will always require preparation and treatment prior to
plastering.
Make sure any surface for PVA is thoroughly clean and free of grease and oils.
Use a product such as Sugar soap to clean greasy painted walls etc before applying PVA.
The background surface should be allowed to dry before applying PVA.
If a surface is dusty PVA can be used as a sealer and allowed to dry before applying another
coat to act as the adhesive.
PVA should be tacky when applying plaster.
Any splashes of PVA on surrounding areas that are not to be plastered should be washed
before they are allowed to set.
Make sure any brushes or rollers used to apply PVA are cleaned thoroughly after use.
BACKGROUNDS AND PREPARATION
SBR Bonding agent
 SBR is a latex based bonding agent suitable for use on external contracts.
 SBR is resistant to water and is not effected by frost.
 SBR is designed to be used with cement based products.
Tiled Backgrounds
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Wherever possible tiles should always be removed prior to plastering.
If they cannot be removed EML sheets should be fixed to the surface using plug and
screws.
Composite walls and dissimilar backgrounds
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A composite wall is a structure consisting of concrete columns with brick or block infill
walls.
This type of structure can cause plaster cracking due to differential movement
If a concrete column is in the middle of the wall, apply heavy duty building paper lapping
onto the brick or blockwork by at least 25mm.
EML is then fixed over the paper with the edges of the metal secured to the brick / block
with nails, staples or plugged screws.
By using this method the reinforced plaster is isolated from the concrete allowing it to
move independently.
Timber wallplates should have a strip of EML or brickmesh fixed with nails to the timber
overlapping onto the brick / block wall by at least 50mm and fixed with nails.
Movement joints
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Wherever a movement joint is placed within a background, movement beads should be used
to bridge the joint.
If purpose made beads are not available place two render stop beads either side of the joint
and plaster up to them. The gap in the middle can they be filled with mastic without
compromising the movement joints purpose.