External Rendering

Rendering External Walls

There has been some debate over what materials to use when rendering external walls. More specifically, some have advocated the use of PVA glue as an adhesive, sealer and bonding agent applied to a cement/ sand mixture
while rendering the external walls of a property (note that in this instance, the term PVA glue refers specifically to the type that is waterproof and can be applied externally like the brands Feb General Purpose PVA or Febond PVA, not the PVA type of glue commonly used to set and bond wooden materials). Others have argued against the use of PVA as a component of the rendering mixture to be applied to the external walls, instead suggesting using a certain proportion of sand mixed with cement (e.g. a rendering mixture of 5 plastering sand to 1 cement with a waterproofing agent added onto the scratch coat followed by a rendering mixture of 4 parts plastering sand to 1 part cement for the second coat).

 It is generally agreed in DIY forums that SBR (styrene-butadiene rubber) is a far superior alternative to PVA when used as a bonding/ sealing agent for cement renders although it is more expensive than PVA. SBR is superior because it is not water soluble when dry compared to PVA. This difference in water solubility is caused by key  chemical and  structural differences between the two compounds which allows PVA to form bonds with the water molecules making it soluble whereas the SBR compound remains unaffected by the water molecules, thus remaining insoluble.

 Other arguments involve the idea that PVA is thought to control the amount of dust present on the background ( i.e. the surface to be rendered)  as well as controlling the suction i.e. the extent to which the water contained within the render mixture (an area of higher water concentration) is drawn towards the background (an area of lower water concentration). The same concept goes for using excessive amounts of water to control the amount of dust present on the background as well as controlling suction. However, according to moderators from the site “plasterersforum”, they have proclaimed that the idea of PVA to control dust and suction is incorrect as well as the idea of using excessive water.

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