There are quite a few plasters but two main types gypsum based and cement based. The latter are used mainly for outdoors, generally for rendering. Gypsum based plasters are used indoors only, damp will attack a gypsum based plaster and cause it to crumble. Cement based plasters can be used indoors for areas that are susceptible to damp and areas that require a greater thickness of cover. The most common gypsum plasters are browning, bonding, and metal lathing plaster. Modern plasters come already mixed with perlite, vermiculite and several other additives. These additives provide a greater degree of insulation and fire resistance. Other ingredients slow down the setting time and generally make the mixture more workable. Today the only additive we need to add is good clean water. When plastering materials with differing absorption, it is a good idea to key the surfaces to be plastered with a PVA bonding sealer. This prevents the plaster going off at different speeds.