Health and safety
Lime is a caustic substance and can cause burns to the skin and eyes, so suitable PPE must be warn, this includes eye protection, gloves and a cover suite, when mixing you should also where a suitable particle mask to prevent inhalation of the fine dust.
Always read the labels or data sheets provided and before undertaking any works ensure that you have fully assed the job in hand, have all the required equipment and are suitably skilled to undertake the works.
Any existing defective plaster or paint should be removed if the plaster is in a sound and is lime based then it should ideally be left to help preserve the building’s history. Take care not to damage the structure and watch for very thick sections of plaster where dubbing out has been undertaken, as these can fall away and cause injury.
This is required to remove the dust and suction from the background material (substrate) and needs to be controlled 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 so moisture is not sitting on the surface when the plaster is applied.
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, which can result in cracking.
Dubbing out (fill out) may be required to deep hollows or holes to the wall surface with haired lime mortar. Rebuilding of some areas of the wall may be required in walls constructed of cob or stone.
The first or scat coat.
Apply one scat coat of 3/1 unhaired lime mortar to provide a key to cob, brick or stone. The scat coat is a soupy consistency of 3/1 unhaired mortar, applied with a brush or trowel.
Apply sufficient coats of 3/1 haired lime mortar to level the contours of the wall. This should be a max depth 20mm, it is best applied in coats of around 10mm thick, shag will occur if the mortar is applied to thick. Between each haired coat ensure this is scratched in straight or wavy lines horizontally. Haired mortar can be applied as many times as required but each coat to dry before attempting another.
Top base coat
The final float coat should be unhaired to avoid hair showing through the top coat, this should be around 10mm deep this needs to be floated up after being trowelled on to form a flat surface, id hair line cracks appear within the surface, then further troweling is required, until no more surface cracking appears.
The finish coat is a 3/2 mix made from very fine sand and lime putty in either one or two applications, best results are from two application using a wet on wet mix and should be applied around 3mm deep.