Preparing Brick Substrates For Render

The plasterers’ craft is one which shows the individuality of the workman’s touch, an essential feature, a desideratum justly regarded as a matter of high importance at the present time. It is a craft which demands exercise of the brain as well as skill of hand.

In every art or craft a perfect knowledge of the materials used, of their characteristics and applicability to various purposes, is absolutely needed to ensure a satisfactory result for labour expended. The plasterers’ craft forms no exception to this rule.

[Reference: Plastering, Plain & Decorative – 1897]

It is very important for plasterers to correctly identify the type of brick substrate they have to work with prior to any render application. Even though we may think a brick is a brick and they are all the same, this is certainly not the case as each type of brick can and will have differing absorption rates, differing surfaces, different compositions and mannerisms.

It is therefore very wise to exercise extreme diligence at this stage to ensure optimum adhesion and hydration of the render. Some bricks may require a Keycote to not only provide adhesion but to also control the suction rate of the render. Others will only require wetting down prior to the render application to do the same.

Failure to do so will affect the adhesion, hydration process and workability of the render and can potentially be more labour intensive overall.

Below are two relevant sectors for your consideration;

  1. Initial Rate of Absorption
    The suction between the render and substrate is a critical initial factor to ensure sufficient water is retained at the interface for cement hydration. This also applies to the prevention of excessive loss of moisture from the body of the render; namely rapid drying which will result in various forms of early age cracking.It is accepted practice that substrates are ‘wetted-down’ to ensure moisture stability that will promote ongoing hydration and improved adhesion. Differential drying of substrates that have been wetted will result in differential rates of absorption and as such the substrate should be ‘re-wetted’ to achieve moisture stability.
  2. Surface Dampening of the Substrate
    Highly absorbent substrates can ultimately create difficulties in controlling suction and subsequent adhesion of the render to the substrate. In the long term, bond failure may result and the integrity of the wall may be impinged. In an attempt to ensure this occurrence is minimised, the adopted practice of ‘dampening’ has proved successful.After initial preparation, the substrate should be dampened and allowed to dry back to a ‘surface-dry’ condition immediately prior to rendering. This reduces excessive suction yet enables bond to be achieved in the case of weaker and open-textured substrates.[Reference: CIA Document on Solid Plastering]

Onsite Laboratory Guide

If you are unsure of the adhesion qualities or suction rates of a substrate, use this simple test. All you need is to pour some water (about one cup) onto the surface of the substrate.




Water beads up into spheres

Possible silicone surface, bond breakers

Consult Rockcote Technical Team (1)

Water pools

Smooth surface, no suction

Apply Rockcote Keycote (2)

Water disappears quickly

Too much suction

Apply Rockcote Keycote    (3)

Water is absorbed slowly

On non smooth surface

Ideal suction

Ideal to render (4)

1) If the water beads up into little spheres, this could indicate the presence of silicone or bond-breakers. In this situation you may not be able to use Rockcote’s pre-blended renders. Up to date advice must be sought from the Technical Team at Rockcote. If a render is to be used, both the silicone and the bond-breaker must be removed prior to rendering.

2) If the water pools on top of the brick/block, this indicates there is no or very little suction/absorption and the render may not adhere sufficiently to the surface. An application of Rockcote Keycote must be applied.

3) If the water disappears into the substrate almost immediately, there is too much suction and Keycote should be applied. A substrate with too much suction can draw the moisture out of the render too quickly which causes the drying time to decrease but affects the overall curing and the physical bond of the render. Cement relies on a steady hydration process to set and cure. If this hydration process occurs too quickly, the render will not set and cure correctly.

4) If the water slowly absorbs into the brick/block, this indicates an ideal substrate to render over and will not require a Keycote.