Code of practice for the application of cement and cement-lime plaster finishes
1972 Edition

The standard suggests mixing dry cement and sand in specific ratios, followed by the addition of lime putty and water to produce a workable mortar. For a smooth lime-putty finish, a typical ratio is 1 part cement, 1 part lime putty, and 3 parts sand by volume. Rough finishes generally use 1 part cement to 3-4 parts sand without lime putty. It is important to note that mixes with higher cement content reduce workability and should be matched to the substrate's rigidity.

Surface preparation depends on the substrate type: smooth, non-rigid surfaces should be roughened by wire brushing, while harder surfaces like concrete require hacking or bush-hammering to expose aggregate for better adhesion. Applying a spatterdash coat (1 part cement to 1-3 parts coarse sand by volume) can improve bonding. The surface should be moistened prior to plastering to prevent excessive absorption, and masonry joints should be raked to about 10 mm depth. In some cases, a thin cement slurry or wire mesh reinforcement may be applied to enhance keying.

To avoid defects like popping and peeling, controlling moisture is crucial; ensure walls are free from dampness by using damp-proof courses and waterproofing. Background surfaces should be clean, dry, and free from salts, avoiding unwashed estuarine or sea sand that can cause efflorescence. Employ quality materials and skilled workmanship, consider climatic conditions during curing, and maintain structural stability to minimize cracking. If salt contamination occurs, remove affected plaster and apply a suitable base separated from moisture sources.

Salts such as sea-salts and calcium chloride in plaster attract moisture, keeping the plaster surface damp and promoting deterioration. Persistent moisture leads to corrosion of metals like steel, aluminium, and lead in contact with plaster. While matured plaster is generally dry and non-corrosive, damp conditions during initial curing or condensation increase corrosion risk. Protective measures include using coatings on metals, corrosion-resistant sleeves, embedding metals in appropriate plaster mixes, and avoiding plaster application on persistently wet surfaces.

The first plaster coat should be allowed to dry and undergo shrinkage substantially before applying the next, typically about one day in summer and two days in winter. During this period, keep the surface damp but not wet to prevent cracking. The second coat should ideally be applied within five days after the first, with surface leveling beforehand. Each coat requires continuous damp curing for up to seven days using fine misting, avoiding soaking. After the finishing coat, maintain moist conditions for at least seven days and protect the plaster from extreme weather.