Render is an all-over coating on an external wall. Here we discuss the options for patching or the complete rendering of a wall.
If a wall is being completely rendered, the best option is an un-gauged non-hydraulic lime mortar using well-matured lime putty and sharp and well-graded aggregate. This does not require special skills. Use non-hydraulic lime and sand in a 1:5 ratio.
Otherwise, in inexperienced hands, 1:1:6 is a good standard mix for render, ie including some cement. Increasing the cement content will make the mortar too hard and remove the benefits of using lime, but any less than 1:2:9 must be avoided.
If applying more than one coat, always apply the stronger coat on the wall - otherwise you prevent air accessing it and carbonating the lime.
If a chemical set is required, perhaps because of low temperatures or high moisture levels, it is best to use an hydraulic lime.
To patch exist lime render, you must use a similar lime render to do the repair. If the mix is very different, cracks will occur at the join between the old and the new material.
Find out if the render was done with hydraulic lime which sets with water, is non-hydraulic ie sets in air. If it is hydraulic lime, this is easier, as it is mixed with sand and water and applied like a cement render.
If the render is non-hydraulic then you need to make the correct choice. Lime mortar, render and plaster is the same. It comes in two grades; coarse 'stuff' is a 1:3 mix of lime and well-graded sharp sand. Fine stuff is a 1:2 mix of lime and a fine sand. If the existing render has a fairly rough texture then you can just use coarse stuff on its own. If the texture is very smooth, then you will probably need to fill the larger holes with coarse stuff leaving a depression of 2-3mm for a top coat of fine stuff.
As with all patching, ensure that the area to patch is clean of dust and that the edges are neat. Make the area to be rendered very wet, especially in warm weather. Apply the lime as you would any normal plaster. Use a wooden float for fine stuff, as metal trowels and floats tend to bring the lime fat to the surface and make it difficult to work.
You must stop non-hydraulic lime mixes from drying out too quickly; if it dries too quickly then it will crack. Protect the work from direct sunlight. Ideally cover it with some damp hessian and spray it with water regularly. Aim to prolong the drying time for one to two weeks to avoid cracks.
If cracks do start to appear then you can deal with them while the lime render is still wet by scouring over the surface with a wooden float to consolidate it and close up the cracks. You can also rub in some additional mortar.