The traditional use of external rendering is two-fold, namely to weatherproof the property facade and to provide pleasing finishes of various textures. Plastering is a skilled trade and a trained craftsman should be employed as often as possible the reason being there are several skills that can only known to experienced tradesmen. Although, if skilled help is not available it is necessary to know the fundamental principles involved, that is how to ensure the preparation is correct, choose the mix, proportion the materials and apply the plaster.
The volume of portland,lime and aggregate you need varies with the reason for the rendering and the type of the outer wall surface on which it is to be trowelled. A genuine lime render is usually soft and takes time to cure, so it is traditionally mixed with portland to boost its strength and durability and to decrease the period of curing.
the strength is boosted and the curing interval decreases as the amount of cement increases. A pure cement stucco with no lime, the bond is at maximum and the setting period the quickest. Remember though, as the amount of cement is boosted the mix becomes less workable and more difficult to apply. The proportion of sand should not exceed three times the overall total of lime and cement. A stronger stucco should not be applied a porous surface nor should a cement rich final coat be put on to a weak under coat.
For outside rendering on such as dense blocks of poor permeability, the ideal combinations to use are 1 part cement, 1 part hydrated lime with 6 parts sand by volume. In weather prone building walls subject to heavy rain storms the volume of lime may be cut back and the cement increased to make the render harder and less absorbent but make sure the mix is still able to be applied.
On exterior property walls of poor strength and high absorption such as cheaper quality breeze building bricks the exterior plastering should not be more than a 1:1:6 mix as a strong cement mix with little or no lime should not be used as it will crack.
When inside rendering the ideal mix is 1 portland, 1 natural lime and 6 sand; or 1 portland, 2 natural lime and 9 sand; or a natural lime mix only. Don’t forget that the more portland you add will strengthen the mix and also cuts the time needed for curing. For water features such as fishponds only pure cement plasters should be applied to maximise total water proofing.
The weather during plastering and screeding may have a big 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. Never carry out any plastering in frosty weather. In cold weather plaster hardens less rapidly and consequently extra time between coats ought to be allowed. In very hot weather the successive coats must be kept damp and protected from completely setting for no less than two days after application. A long with keeping direct sunlight off the plaster a gentle draught from an open roof hatch will keep the render from setting 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