The type of background on which the rendering is to be applied will influence the choice of material to be used and the methods of preparation and application. The types of background include brick; burnt clay block; poured concrete or concrete block, with dense or lightweight aggregate; aerated concrete; no-fines concrete; and lathing, usually metal, fixed on a framework or grounds. There are 5 categories of backing. (1) Dense, strong and smooth material- it may sometimes be difficult to get renderings to adhere properly to such materials as dense concrete, either precast or cast in place because of their smooth surfaces and low porosity. A mechanical key provided by hacking during the manufacture of the units can be a useful safeguard with such surfaces. (2)Moderately strong porous materials- the more porous but moderately strong materials, including most clay block, and concrete and clay block, have fairly high suction and afford a good mechanical key for renderings. (3) Moderately weak porous materials- the weaker porous materials, such as concrete made with lightweight aggregate (pumice, foamed slag, clinker, etc.), aerated concrete, and some rather soft types of brick, generally offer no difficulty in application or adhesion of a rendering. (4) No fines concrete- no-fines concrete is peculiar in that it has a large proportion of voids but has practically no suction. The open structure, however, gives a good mechanical key and the material is strong enough no to be broken up by shrinkage stresses if the thickness of rendering applied is not too great. Finally, (5) metal lathing- in using metal lathing, the more important points influencing the choice of rendering material and finish are the risks of corrosion of the metal and of cracking of the finish. Portland cement and lime both help to protect steel against corrosion, the protection being rather greater with mixes richer in cement, owing to their lower permeability to moisture. The materials used in external renderings are cement, lime, and fine aggregate. Each of the materials is important, but it is probably more difficult to insure that the aggregate is suitable than to insure good quality cement or lime. Generally it is advantageous for scraped, smooth or roughcast finishes to use sand as coarse as can be applied. Crushed stone sands for renderings should comply with the same requirements, although some recent work suggests that crushed limestone sand may contain higher proportions of fine material without causing any trouble.