External Rendering

Curing Externally Applied Rendered Surfaces And The Problems Associated With It

Lime putty based render is known to be less flexible than hydraulic lime render and has the longest time duration to cure/ dry, requiring a minimum of three months. Lime putty based render requires a long time to cure because it slowly absorbs carbon dioxide (CO2) from the atmosphere, undergoing a chemical change from calcium hydroxide (Ca(OH)2) to calcium carbonate (CaCO3) following carbon dioxide absorption resulting in the render hardening.

Note that while the lime putty based render is undergoing the process of hardening, no frost is to be present or else the plaster will be damaged due to the frost slowly undermining the structure of the plaster.

In the case of solid backgrounds such as brick or stone, once the wall is prepared, repainted and given a minimum coat of applied render, animal hair is applied to the render. Formation of a non-cracked hard background is achieved by allowing the render sufficient time to slowly cure as well using float tools to prevent the plaster from slipping off the surface to which it is applied.


The problems associated with the use of pure cement mortars  (i.e. not allowing moisture to pass freely through the structure of the property and instead allowing a build up of moisture, leading to gradual wearing of the structure) caused many builders to use combinations of cement and lime to incorporate lime’s water permeable properties into the diluted cement mortar in the following proportions: cement/lime/sand in1:1:6 and 1:2:9 but still found problems with this diluted form of cement render. Hence the other option was to use the less traditionally made lime putty or hydraulic lime mortars containing excessive water.

Good quality lime putty of this type is one that is several months or years old that has been punched through a sieve rather than ‘poured’ through to remove any uneven shapes or large unslaked (lime that hasn’t been completely dissolved by water) or underburnt particles of lime and, even with pozzolanic additions (porous ash which reinforces the render mix and helps set the render even in wet conditions) requires long curing and protection measures. Its characteristics make lime putty suitable for interior finishes and, at best, exterior work in sheltered areas.

Note: Exterior work involving the use of lime putty or hydraulic mortars should be completed well before the Autumn/Winter months otherwise it should be protected during the Autumn and Winter period.


Pure and natural hydraulic limes (NHLs) made with traditional methods such as using a kiln to burn the limestone do not contain chemical components of cement so they set quickly, have shorter curing times and are easy to mix, even in conventional mixers



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