External rendering is the term used to describe the application of a smooth external finish to walls of a building that are constructed from concrete blocks. Render itself is a mortar or bonding agent consisting of cement and fine aggregate.
Traditionally, render has been made from sand, cement and lime or a plasticiser--a substance that makes the render more workable--and finished whilst still wet. Painting render to make it more attractive and protect it from the weather is one possibility, but self-coloured cement renders are also available.
External rendering is applied to exterior walls to reduce the amount of moisture that penetrates into the underlying masonry. It is also used to enhance the appearance of plain masonry walls.
Considerations for external rendering include the type of substrate, or base material, to which the render is applied, the level of exposure to wind and rain, and the desired appearance of the finished exterior. These factors, in turn, dictate the composition of render and the number of coats that need to be applied.