Render and Finishing Systems

About this course.

Render is a coating applied to a wall to protect and finish it, and the main types differ chiefly in their binder — the ingredient that sets and holds the render together. The short answer is that traditional renders are bound by cement and/or lime and are essentially mineral coatings, while modern acrylic and polymer-modified renders use plastic polymers, either alone or blended with cement, to gain flexibility and adhesion. Each family behaves differently on the wall, and choosing the wrong one for the substrate and exposure is a frequent source of problems.

Traditional cement and sand render is the workhorse of the trade. It is a mix of cement, sand and water, often with lime added, applied in coats and worked to a flat or textured finish. Its strengths are hardness, availability, low material cost and a long track record. Its weaknesses are that it is relatively rigid and brittle, so it can crack if the wall moves or if the mix is too rich in cement, and it has limited ability to accommodate movement or to bond to smooth, dense or non-absorbent surfaces without help. A cement render that is stronger and less flexible than the wall it covers tends to crack, because the wall's movements are transferred into a coating that cannot stretch.