Understanding the nature of the substrate (the wall to be rendered) is critical to the success of any
rendering job. The key properties of the substrate are:
Water absorption or ‘Suction’
In general, stronger substrates will require stronger render mixes. Whilst a degree of suction is required to enable the scratchcoat to bond to the substrate, excessive suction (e.g. in aerated concrete blocks) may draw too much water out from the scratchcoat or any applied spattercoat. The water molecules will migrate towards the substrate due to the observed random movement of water particles from an area of high water concentration (in the scratchcoat/ spattercoat) to an area of low water concentration (in the porous substrate material). This in turn will inhibit cement hydration (particularly with General Purpose Cement and Mastercrete which are relatively slow setting) and cause a significant reduction in both the bond and the strength of the scratchcoat and/or spattercoat. Excessive suction can usually be prevented by dampening the surface of the substrate before applying the scratchcoat as this will reverse the movement of water particles, they will instead migrate away from the porous substrate because the water concentration has increased as a result of the substrate dampening.
Suction is a major factor in determining how good the adhesion of a render is to a substrate. How good the adhesion of a render is to a substrate is in turn dependent on the porosity of the substrate. If the suction is too high, the render will hydrate sufficiently. If the suction is too little, there will not be a good bond.
Good porosity of the substrate is very important for the render to achieve a good bond. If the substrate is too porous, it will not allow the render to hydrate sufficiently and if the substrate is to a small degree porous, there will not be a good bond. In these extreme situations, the following solutions are:
For high porosity substrate like in the case of low density blocks, the substrate should be lightly sprayed with a fine mist of clean water evenly before the render is applied. Saturation should be avoided as this will cause excessive shrinkage.
For low porosity substrate like concrete or bricks, a render should be applied that manipulates the level of porosity.
Examples of porous substrates are concrete and wood. Examples of non porous substrates are ceramics.