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As with all lime
putty based materials the best outcome requires patience and careful control of
drying and suction, the reward being a good looking and long lasting plaster.
We also supply a range
of earth plasters for use in traditional properties and new build. They
are designed to be used over cob, clay boards and reed mat.
Before
starting any work, always check that the building is not listed, if it
is contact the local authority conservation officer to discuss the project.
Preparation:
Any existing
plaster and paint should be removed. If there are sound lime mortars these
should ideally be left to help preserve the buildings history and it clearly
reduces material costs. Care must be taken to ensure that the structure
isn't damaged. Look out for very thick patches of plaster that are effectively
load bearing. It may be necessary to plaster on top rather than risk rebuilding
an area.
Damping:
It is very important
to control suction from the background material (substrate) by damping
with water before applying each coat of plaster, especially onto cob or
porous brick. Although control of suction is important, so is surface
tension so any damping down must be allowed to soak in and not be sitting
on the surface when the plaster is applied.
Premixing:
Generally, lime
putty mortars and plasters benefit from being pre-mixed for a minimum
of a couple of weeks and then "knocked up" again prior to use to plasticise
them - this reduces shrinkage in the plaster. The pre-mixed and ready
to use materials can be purchased from Mike Wye & Associates Ltd.
or our outlets.
Example Specification
on to solid walls:
- dub out any deep
holes in the wall with a haired lime putty mortar, rebuilding defects
using like for like materials such as cob blocks, cob bricks or stone.
- treat wooden lintels
with preservative and counter batten with oak lath if they are to be
plastered over.
- apply one hand
harled coat of 3/1
unhaired lime putty mortar to provide a key to cob or brick. With
stone this is a matter of preference depending on the friability and
size of stone and joint if this stage is ignored.
- apply sufficient
coats of haired lime putty mortar, 3/1
haired lime putty mortar, to smooth the contours of the wall, the
last float coat may benefit from being unhaired as then there is no
chance of hair coming through the top coat.
- apply a top coat
of our lime-rich, 3/2
lime putty plaster, based on a very fine sand and lime putty in
either one or two applications. For the very smoothest of textures,
the second coat should be a 1mm skim of our Regency
lime putty plaster.
Gauging:
If there is
a high residual level of moisture in a wall that cannot be eradicated
(eg a high external ground level) it may be necessary to sandwich a waterproof
barrier between the coats of lime mortar. We supply a waterproofing slurry
for this purpose. Backing coats of lime mortar can be gauged with Argical
M1000 or NHL5 (eminently hydraulic lime) to get an hydraulic set, the
latter being especially useful for the harled coat. Argical
M1000 or natural hydraulic lime (NHL) can be added to the top coat
plaster to ensure extra durability where knocks are expected, 10% Argical
or NHL5
by volume.
Plaster Skim:
Our Regency or 3/2 fine lime plaster is ideal for a thin skim over a wide
variety of backgrounds such as plasterboard, blocks and mixtures of old
and new plaster, old paint etc. For plasterboard it will be necessary
to apply a special bonding coat DG27.
For bare plasterboard, it is always necessary to scrim the joints as is
usual practice, normally this would be applied with a very thin coat of
gypsum although lime
wall finish smooth could be used. Failure to carry this out may result
in cracking at the joints. Our lime plaster 3/2 can then be applied in
one or two very thin coats or Regency plaster for the second coat for
the very finest of finishes.
Quantities:
for
an Example Specification per square metre on to a rough undulating surface
one scat coat of
3/1 unhaired mortar 6kg per m2 (3mm)
one scratch coat 3/1
haired lime putty mortar, 30kg per m2 (15mm)
one float coat 3/1 unhaired
lime putty mortar, 20kg per m2 (10mm)
two top coats
of 3/2 lime
putty plaster, totalling 6kg per m2 (3mm)
Safety:
Limes
are caustic. Always wear eye protection and protective gloves and clothing and
follow the safety instructions on the labels. Our
advice and information are given in good faith. It's important that users satisfy
themselves that they've chosen an appropriate product and have a suitably skilled
workforce. Back
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