Masonry - Finishes
Certain types of concrete masonry can be left unfinished while others will readily accept an applied finish, provided it is suitable for application to an alkaline background and permits a degree of moisture vapour movement through the construction.
Paint: A suitable paint for application to concrete masonry should be specified. Some blocks can and are intended to receive paint directly. Others may require the filling of small blemishes if a smooth appearance is required.
Plaster and render: Gypsum or sand /cement based plaster can be applied to internal concrete masonry. External use would require the application of sand/cement render. A mechanical and/or chemical key will be required to enable good adhesion between the plaster/render and blockwork background. Many standard textured aggregate blocks have such a mechanical key. Some preparation may be necessary with low suction and smooth backgrounds, e.g. raking of the joints or the application of a bonding coat. Also, adjustment to the consistence of the mix may be required with high suction backgrounds to prevent rapid loss of workability. Unless a proprietary system is used, renders are normally applied in two coats. The finishing coat should be thinner and/or weaker than the undercoat. The finish required would dictate the render mix but strong mixes should be avoided with low strength backgrounds.
For further information see the Concrete Block Association or the Autoclaved Aerated Concrete Products Association