t
Plastering Hydraulic Lime Plaster
Preparation of the Wall Surface
The successful application, bonding and correct hardening
of hydraulic lime mortars, requires that the background should
be clean, free from vegetation, free of containments and reasonably
dry throughout the wall mass. The wall should be structurally
sound and the masonry and bedding mortars in good condition.
The application of various dubbing out coats should not be
seen as remedial repair work to the masonry or brickwork background,
the replacement of missing or damaged masonry or open joints
should be repaired before any plastering of dubbing out coats
are applied. Where excessive hollow or unevenness is present
the areas should be corrected by use of mortar and stone or
brick slips bedded into the mortar, with the aim of presenting
a reasonable flat background before plastering commences.
Suction Control and Bonding
Before the application of any new lime coatings, hydraulic
or non-hydraulic, it is vitally important to check to the
degree of suction within the background, poor or excessive
suction can result in a weak bonding with the substrate caused
by rapid drying of the newly applied render, which will result
and a weak and powdery interface which will lead to later
failure and separation. Where there is little or no suction
further action will be required to help bond the coating to
the substrate. In situations where suction needs to be controlled,
wetting down will be required, on dense blocks or near impervious
masonry, simply dampening the surface with a mist spray may
be all that is required, but on very porous surfaces such
as old brickwork considerable wetting will be required. Wetting
the wall by use of a hose, working from the top of the structure,
downwards, may need to be carried out the previous day or
several times throughout the day before rendering commences.
The objective of the suction control is to achieve a thoroughly
damp surface, but not wet, i.e., the surface must not have
running or standing water remaining on the masonry or brick,
this will form a barrier between the coating and substrate,
also lime mortars adhere and stiffen through a certain amount
of suction.
On dense or near impervious background, it may be necessary
to apply a sand/lime splatter dash coat to the background
to act as a mechanical key.
Salt Contamination
Where new lime coatings are to be applied to masonry which
is salt contaminated, the masonry should be allowed to dry
fully before applying new renders. This will allow salt to
be detected on the masonry and mortar joint surfaces, if excessive
salt is identified clay or lime mortar poulticing may be required.
Specialist advice should be sought, if the technique is considered
where salt is detected on the mortar joints, rake out the
joints to a depth of 50mm, as this is likely to be heavily
contaminated and in a weakened condition and re point (See
re pointing).
Salt contaminates should never be washed from the surface,
as this will result in the crystallized salt returning to
a soluble state and retreating back into the pores of the
masonry or brick. Where detected on the masonry surface, the
salts should be brushed from the surface and cleaned away
from the structure.
Techniques of Hydraulic Lime
Plastering
The techniques employed in the application of hydraulic lime
plasters should be to ensure a correct bonding with the background
while striving to minimise shrinkage and rapid drying, these
techniques should be followed throughout the plastering process.
Lime plastering is generally applied in 3 coats, but it is
common to find 2 coats or even single coat work in vernacular
or early structures. In 3 coat work the first coat on masonry
or brickwork is generally known as the scratch coat or render
coat, this coat is applied in a coat of approximately 10mm
thickness, this coat can be applied by use of a steel trowel
or thrown onto the wall by use of a harling trowel and then
flattened in by the steel trowel. When this coat has firmed
up but has not gone hard, the plaster is keyed or scratched
up to produce a key for the following coats. The keying up
is carried out by use of a lath scratcher or similar toothed
instrument and care should be taken not to cut through the
plaster coat back to the background.
The keying up is generally in the shape of diamonds of approximately
30mm spacing. This coat should be allowed to harden for 72
hours minimum before further coat of plaster are applied,
before applying the second coat the first coat should be checked
for shrinkage cracks, and these should be filled with plaster
before proceeding with further coats, before the second coat
is applied the first coat should be brushed down to remove
any dust which may have blown onto the surface. The first
coat should then be damped down to ensure that the second
coat is applied to a damp but not wet surface. This second
coat is called the floating coat and is the coat which is
straightened to ensure a flat and even surface, after this
coat has been straightened, the surface of this coat is scoured
up with a timber or polyurethane float.
The purpose of the scouring is to compact the plaster and
counteract shrinkage. This process is generally required to
be carried out at least twice, during the scouring up, any
small holes should be filled before the finishing coat is
applied. This coat should also be allowed to dry and harden
for 72 hours before applying the final coat.
After once again damping down the floating coat, the final
coat is applied. This coat known as the setting coat, is applied
in two layers, coat on coat, with the trowel. This coat is
applied in a way similar to skimming. This material should
be of a wet enough consistency to allow a long and even spread.
Once the setting has been applied, it is scoured in a similar
manor as the floating, to counter-act shrinkage and then the
surface can be left with a rubbed up rendered type finish
or polished smooth with a steel trowel.
General Mix Guide
Backing Coats 1st and 2nd
1 Part NHL2 or NHL3.5 to 2Parts or 2.5Parts Washed Sharp Sand
Use of fibre or hair in these coats is recommended
Finish Coat
1 Part NHL2
1 Part Fine Silica Sand
Note
Highly polished surfaces will not allow the passage of moisture
as well as an open textured surface.
Return To Top
The Building Lime Company | Tel: 01980 611950 | Fax: 0845 658
7788 |