The purpose of rendering is two-fold, namely to weatherproof the building wall and to provide pleasing finishes of various textures. Plastering is a skilled trade and an expert plasterer should be employed wherever possible, as there are many points that can only be learnt by experience. However, if skilled help is not available, it is necessary to know the elementary principles involved, that is, how to prepare the surface, choose the mix, proportion the materials and apply the plaster.
The proportions of cement, lime and sand to use depend upon the purpose of the plaster and the nature of the building surface to which it is to be applied. A pure lime plaster is comparatively soft and weak and is slow setting, so it is usually gauged with cement to increase its strength and hardness and to shorten the period of hardening. The strength increases and the period of hardening decreases as the amount of cement increases in proportion to the amount of lime, until with a pure cement plaster with no lime, the strength is a maximum and the hardening period the least. However, as the proportion of cement is increased the plaster becomes less workable and more difficult to apply. The proportion of sand should not exceed three times the combined proportions of lime and cement. A stronger plaster should not be applied to weak porous backing or a strong finish coat to a weak first coat.
For external building plastering or ‘rendering’ on a dense material such as concrete, dense concrete blocks and hard clay bricks of low porosity, the best proportions to use are 1 part cement, 1 part hydrated lime and 6 parts sand by volume. On exposed building walls subject to driving rains the proportion of lime may be decreased and the cement increased to say 1 ¾:¼: 6 mix or a 1:4 cement plaster to which up to 10% of lime by weight of the cement is added to make it workable.
On external building walls of low strength and high porosity such as low quality breeze concrete blocks the external rendering should not be stronger than a 1:1:6 mix as above and a strong cement mix with little or no lime should not be used.
For internal plastering the best proportions are 1 cement, 1 lime and 6 sand; or 1 cement, 2 lime and 9 sand; or a lime mix only of 1:3 or 4 may be used, remembering that the cement increases the strength and hardness and reduces the period of hardening. For water containers such as fishponds only cement plasters should be used.
The weather during plastering and rendering may have considerable influence on the finished work. External rendering should be started on the shady side of the building to keep it unexposed to the sun as long as possible. No plastering should be done in frosty weather. In cold weather plaster hardens slowly and a longer period of time between coats should be allowed. In very hot weather the successive coats must be kept damp and protected from drying out for at least two days after application. Draughts and local sources of heat may cause too rapid drying out. Gentle heat and moderate ventilation are the best conditions for plastering.