Gypsum plasters

Gypsum plasters are factory produced, pre-mixed plasters with lightweight expanded minerals as the aggregate and gypsum as the binding agent. They require only the addition of water on site to make them ready for application. In addition to the qualities of sanded gypsum plaster (see above), they provide the following additional advantages, they:

   -are easy to mix and no aggregate has to be added on site
   -improve thermal insulation
   -help to mitigate the effects of condensation
   -are light and therefore easy to apply
   -have good fire resistance.

These plasters have proved very popular over the past 40 years, though in the past ten years or so their use has diminished significantly as the house construction industry has sought prefabricated and dry-finishing solutions such as dry-lining (see later section).

Gypsum plasters are generally applied in two coats and manufacturers produce a range of undercoats and top coats designed to suit different circumstances.

Undercoats have the addition of coarser aggregates than finishing coats and are designed for specific backgrounds and for practically every requirement. For example:
   -Browning: works well on solid backgrounds, such as block or brick with moderate suction and adequate mechanical key (dense concrete blocks or bricks with well raked-out joints).
   -Bonding: works well for low-suction backgrounds, such as smooth dense concrete or plasterboard.
   -Toughcoat: an undercoat with good impact resistance for most backgrounds including metal lath.
   -Hardwall: can be applied by hand or machine. It is suitable for most backgrounds, except plasterboard.

Finishing coats contain finer ground aggregates and gypsum than the undercoats to give a smooth, even finish. Three types of finish are typically manufactured:

   -a fine finish which is suitable for plasterboard
   -a final coat plaster suitable for gypsum undercoats
   -a final coat plaster suitable for a range of backgrounds (i.e. lime or cement or gypsum-based undercoats).

In the more recent past, a number of single-coat plasters have been introduced; largely, these are designed to be machine projected, rather than hand applied.