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Guidesheets: Plastering
Lime putty plasters offer advantages over cement based mortars and pure gypsum plasters for the internal plastering of traditional properties, especially when decorated with a traditional limewash.
- their porosity allows the structure to "breathe".
- they can accommodate general movement better.
- their self-healing nature reduces cracking problems.
- they can reduce condensation problems.
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 and include a range of self-coloured finishing plasters that don't require painting.
Preparation:
Any existing plaster must be removed, except any sound lime mortars. Care must obviously 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 spraying with water before applying each coat of plaster, especially onto cob or porous brick.
Premixing:
All lime 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.
Example Specification:
- dub out any deep holes in the wall with a haired lime putty mortar, rebuilding defects with cob blocks, cob bricks or stone.
- treat wooden lintels with preservative and counter batten with oak lath.
- apply one hand harled coat of lime putty mortar, 3/1 unhaired to provide a key to cob. With stone this is a matter of preference depending on the friability and size of stone and joint.
- apply sufficient coats of haired lime putty mortar, 3/1 haired, to smooth the contours of the wall.
- apply a top coat of our lime-rich plaster, 2/1 unhaired, based on a very fine sand and lime putty.
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 Metastar or NHL5 (eminently hydraulic lime) to get an hydraulic set, the latter being especially useful for the harled coat. Gypsum, Metastar or natural hydraulic lime can be added to the top coat plaster to ensure extra durability where knocks are expected, up to 2/1/1 sand/lime/gypsum or 20% Metastar or NHL5 by volume.
Plaster Skim:
Our 1/1 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 gauge the lime plaster with gypsum onto a drying initial thin coat of pure gypsum. We can also supply a special bonding coat for plasterboard called DG27.
Quantities:
for the Example Specification
per square metre
one scat coat of 3mm - 0.3 tubs of 3/1 unhaired mortar ~ 21kg size
two dub coats each of 12mm - 2.4 tubs of 3/1 haired mortar ~ 21kg size
one top coat of 3mm thickness - 0.3 tubs of 2/1 plaster ~ 20kg size
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.