The reason for exterior rendering is two-fold, chiefly to shield the property structure also give attractive finishes of various styles. Screeding is a difficult trade and an expert plasterer should be hired whenever possible,the reason being there are several skills that can only known to experienced tradesmen. Although,if skilled help is not easily obtained,it is crucial to know the elementary techniques involved, basically,how to prepare the surface, select the mix, proportion the materials and apply the plaster.

The amount of portland,lime and sand that are required differs the purpose of the plaster and the nature of the masonry surface to which it is to be applied. A authentic lime plaster is comparatively soft and is slow setting, so it is traditionally mixed with cement to add extra strength and durability and to shorten the period of hardening.
the strength improves and the curing interval lessens as the amount of cement rises. A 100% cement plaster with no lime, the mix is at its peak and the hardening period the quickest. However, as the volume of cement is increased the plaster increases in density and harder to plaster. The proportion of sand should not exceed three times the overall volume of lime and portland. A cement rich stucco should not be applied a blown backing nor should a cement rich final coat be trowelled on to a less strong under coat.

For external rendering on such as hard clay bricks of low porosity, the ideal combinations to work with are 1 part cement, 1 part natural lime with 6 parts sand by volume. In exposed house walls subject to driving rains the amount of lime may be decreased and the portland boosted to enable a heavier mixture and less absorbent but check the mix is still workable.

On outside building walls of weak strength and high permeability such as lesser quality breeze concrete bricks the exterior stucco should not be stronger than a 1:1:6 mix as a strong cement mix with little or no lime should not be used as it will crack.

If inside rendering the optimum combination is 1 cement, 1 hydraulic lime and 6 sand; or 1 cement, 2 natural lime and 9 sand; or a hydraulic lime mix only. Don’t forget that the more cement you use will strengthen the mix and also decreases the time needed for curing. For water features such as fishponds only 100% cement plasters should be applied to maximise complete water proofing.

The conditions during plastering and screeding may have a big influence on the finished work. External rendering should be begun on the shady side of the building to keep it protected from the sun as long as is practicalpossible. No plastering should be done in frosty weather. In cold weather plaster sets slowly and a longer period of time between layers ought to be allowed. In extremely high temperatures the different layers need to kept damp and prevented from drying out for a minimum of two days after application. In addition to keeping direct sunlight off the wall a mild draught from an open door will keep the render from curing too quickly.

Joe Smith has a website and Blog featuring tips and advice about external rendering and render painting. It explains what is best practice for rendering walls