Masons Mortar Ltd
77 Salamander Street
Edinburgh EH6 7JZ

Tel: 0131 555 0503
Fax: 0131 553 7158

Masons Mortar Ltd
Unit I, Woodville Court
Woodville Street
Glasgow G51 2RL

Tel: 0141 445 4812
Fax: 0141 445 8898

Masons Mortar Ltd
Block 2
Woodend Industrial Estate
Cowdenbeath, Fife

Tel: 01383 514 460

Masons Mortar Ltd
Unit 11
Brough Park Trading Estate
Fossway, Newcastle upon Tyne

Tel: 0191 908 9234

Lime Plaster
Lime Plaster
This is not a specification or a set of instructions. It is for information only. The sole purpose of which is to provide the reader with general information related to the use of products supplied by us.

Plastering in ordinary mortar normally consists of three-coat work, using traditional materials. Lime plaster was generally made with non: or feebly hydraulic lime and sand and this is the basis for this guide. This type of lime sets and hardens predominantly by slow evaporation of water from the mix and re-absorption of Carbon Dioxide from the air. By its nature the drying: absorption process is slow, therefore lime plaster curing can be a lengthy process, and should not be hurried.

For plastering on the hard, the background will normally be brick or stone. The surface should be clean, free from dust and any organic materials such as lichens etc. Test the surface of masonry backgrounds for dust by applying a piece of masking tape to the background and immediately remove, examine the sticky side for traces materials that may affect the bond between the plaster and the wall

Internal walls can be uneven and rough, often with areas that have been altered. Different background conditions are therefore common and this needs to be addressed before plastering. Deep holes, wide joints or pockets should be dubbed out in thin layers of mortar with pinnings (bits of brick or stone) tightly bedded in mortar and left to cure. The aim of preparing the background should be to achieve a surface that can take a first coat of consistent thickness, and to provide an adequate key for this first coat. The quality of preparation work is vital to the quality of the finished job. Suction between the first coat and the wall (and between all subsequent coats) is the primary means of bonding although a physical is also important. Different materials have different levels of suction, so for instance where a door way has been knocked through a stone wall and the edges built in brick, the brick may well have a different level of suction to the stone. Understanding and controlling suction is important for successful work.

For lath and plaster work, laths should be fixed by butt and break joints to joists or battens securely fixed back to wall or ceiling, with gaps between the laths of approx, 8 - 10mm. The support battens or ceiling joists should be spaced such that the lath does not give unduly in the centre. Wide spacing of battens or joists may require intermediate support or thicker laths used. Laths, sawn or riven (traditionally hand made) should be thoroughly damp before fixing. Dry laths swell when wet mortar is applied to them, sometimes causing the laths to bow, in or out. Nails for fixing lath should be thin shank to avoid splitting the ends. Building paper and insulation is occasionaly placed between laths and outside walls to comply with current building control requirement's, this will have a dramatic effect on the drying rate and prevent proper rivet formation when fixed hard against the back of the lath. If building paper and insulation are essential, use moderately or eminently hydraulic lime for the first coat as it has a natural set and does not depend on slow drying, maintain at least a 20mm gap between the paper and the lath.

First coat, Render coat.
Coarse stuff is knocked up immediately before use by beating the coarse stuff to increase the workability. It may be necessary to add water to the mix at this stage. Immediately prior to use, hair is teased into the mortar and repeatedly chopped and turned until a good even distribution is achieved. The hair should be long and strong, free from lumps and clumps. It should be clearly visible as a beard around the edge of an inverted trowel full of mortar. The hairs should be at approximately 1mm centres.

When working on the hard the first coat of mortar should be applied by throwing (a spray gun can be used) or with a laying-on trowel on to a dampened but not wet background at approximately 9 - 12mm thick.
For application onto laths it should be trowel applied as evenly as possible and pressed home to form rivets between and behind the laths. The render coat should not be straightened.
As initial shrinkage takes place in the drying out phase, tighten the entire surface in with a cross grained wood float, closing back by further dampening and tightening in a close circling motion.
Key the surface with a comb, or with a lath scratcher for plaster on laths, taking care not too score to deeply, which will cut the hairs and may cause a loss of integrity in the coat. In the case of lath work ensure that scratching is across; not in line with the lath. Thereafter if necessary control the rate of drying out by misting the surface with clean water or lime water, until all shrinkage has stopped and the mortar has hardened sufficiently to receive the second coat. Do not over- wet the surface. (When beads of water appear on the surface it is over-wet.) Plastering on laths may take longer to dry as there is no suction of moisture into the background.

Second coat. Floating coat. (This is the straightening coat)
Using the same coarse stuff as the render coat and following the same method of knocking up and introducing hair, apply with a laying-on trowel to a dampened background. At this stage if straightening is necessary then fill out deepening's in thin layers and apply the floating coat to the desired line using rules or dabs. Maximum thickness should be 12mm in one pass. Tighten in as before with a cross grained wood float and key lightly with a devil float to receive the finish.

This coat should be finished to a high standard, straight and true, ready to accept the finishing coat (setting stuff)


Setting Stuff. Third coat. (Finishing coat. Hair is not required)
Apply with a laying-on trowel in two passes to an overall thickness of about 3mm maximum, less is better and tighten in with a cross grained wooden float, dampening as necessary as the set takes up. Setting stuff sets by suction from the background. The final finish is produce by troweling the surface with a steel float and dampening as necessary.
Patching finished lime plaster into a repair requires care. The finish is sand based and will abrade and degrade existing edges leading to dull open surfaces around the repair. A clean break of a few mm is necessary around the patch that can be filled later with lime putty when the patch is fully hardened

Wallpaper or paint finishes should only be applied to fully dry and set work. Lime wash, distemper and casein paints are traditional finishes for new work. Laitance reduces suction of new plaster and will prevent lime wash adhering to the surface. In these instances and only when the work has fully dried out, a fine abrasive paper can be used to improve suction. The surface should never be abraded such that the finish is damaged.

Work should never be undertaken in frosty conditions or where the temperature is likely to fall below 50 C during the execution of the works or until the mortar has hardened. Protection should remain in place for as long as possible. Ensure that the rate of drying is consistent and that strong draughts are excluded from the working area. This is particularly important where a building has windows removed or doors open. Never force the drying by introducing heaters, heating in the building should be background only and should be consistent. If heating is required to maintain a proper working temperature, use Propane heating, this has the effect of producing both moisture and heat simultaneously. Ensure the temperature is adequately controlled. (Any heating appliance should be used strictly in accordance with manufacturers recommendations)

Good working practice.
Adequate preparation and protection are essential. Due care and attention should be applied to all the works. Sample panels should always be carried out in accordance with the specification. Three-coat work will require sufficient time to allow for properly executed and finished samples.