A traditional cement render mix is comprised of;
- Clean sharp washed Sydney sand
- Grey general purpose cement, AS3972 (*1)
- Hydrated plasterers lime, AS1672.1 (*2)
We use this mix for cement rendering over Bricks both dry press and extruded and over Concrete and Concrete Blocks.
In the case of Concrete, Concrete Block and some extruded bricks it is essential that a Key or Splatter coat is applied and allowed to cure prior to applying the Cement Render coat, this is done to prevent any chance of cracking or drumminess occurring, as these substrates do not offer adequate adhesive qualities.
Key or Splatter coats consists of Cement, sand and an adhesive component, either a pva, acrylic or latex additive mixed to the appropriate proportions depending on the condition of the wall.
Before applying any coatings to walls they must be diligently cleaned to ensure no contaminants remain that may prevent proper adhesion. Depending on the wall’s condition it may require as little as a light rinse with tap water or for more stubborn contaminants pressure cleaning may be required, in some extreme cases there may be the need to use detergents or algcides.
*1; We only use Grey General purpose Cement, we never use Builders Cement as it is not suited to Cement Rendering, though many contractors use it because is cheaper than General purpose, some common problems such as lower strength, higher incidents of shrinkage cracks, leaching of contaminant colours through the render and paint may occur.
*2; We only use Hydrated Plasterers Lime in our mix to plasticise, it is the only plasticiser which actually assists in the curing process, promoting higher strength and reducing the incidents of shrinkage cracks and therefore drumminess. Many contractors prefer to use cheaper plasticisers as a substitute for plasterers lime, these are mostly clay based and over time exhibit excessive Expansive and contractive characteristics leading to cracking and ultimately drumminess, classic symptoms are the cracks which develop over time commonly referred to as spider webbing or egg shelling. These products are generally used by contractors because they are cheaper than plasterers lime and because they are more user friendly to the lower skilled Tradesperson.