External Rendering

Why does a cement render coated with a conventional paint system crack?

All cement renders irrespective of how they are formulated will eventually develop surface cracks due to the nature of cement curing and hardening. Sand and Cement renders are highly alkaline, rigid and brittle surfaces that develop surface cracks through cement shrinkage and thermal expansion & contraction from daily temperature fluctuation.

Sand and Cement Render is conventionally painted for aesthetics. Decorative conventional paints are a low film build meaning that this technology is not formulated to withstand the stresses caused by thermal expansion and contraction of cement based rigid substrates. This in turn forces the paint film to crack due to insufficient film build. Lack of flexibility (elongation) leads to poor crack bridging to withstand the substrate dynamics. These cracks over time will continue to expand & contract and become a gateway to water, corrosive and atmospheric pollutant attack potentially leading to delamination and/or ‘drumminess’.

Traditionally “cement render” means site mixed sand and cement (bulk materials mixed on site) but increasingly these days includes various supplier pre-packed bagged cement renders. Pre-packaged bagged supplied cement renders offer some advantages over conventional site mixed render in terms of ease of handling, projectability via machine and product consistency of mix formula (ie. vs uncontrolled site mixed render), water repellancy, and being ‘breathable’ (ie. through coloured and therefore not painted).

Conventional site mixed render in not waterproof nor is flexible in any shape or form and therefore is prone to cracking over time. All sand cement renders once cured form part of the initial building substrate then a finishing coating system is applied for decorative and protective purposes.

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