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 Internal Natural Hydraulic Lime Plastering

 

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.

Cor Castle interior 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.

Cor Castle interior

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 (rubbing up with a float similar to finning up render).

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...