From DIYinfo.org
The main constituent of ready-mixed plasters is gypsum, calcium sulphate, which has been partly or wholly dried in a kiln. The extent to which the gypsum has been dried, and the addition of further constituents during manufacture, determine the type and grade of the plaster. However, as water is detrimental to gypsum plasters after they have set, they should not be used for external work.
Gypsum plasters, incidentally, should not be confused with cement render, a surfacing material made from sand, cement and lime and generally used externally. Render looks and feels like bricklaying mortar, which has similar constituents.
Lightweight, pre-mixed gypsum plasters are the most commonly used nowadays, by both professionals and amateurs. They come in several types, each used for a specific purpose.
Browning is a floating coat (undercoat) plaster used on semi porous surfaces such as bricks, clinker blocks (breeze blocks) and concrete blocks.
Bonding is a floating coat plaster used on less porous surfaces such as poured concrete, where getting good adhesion is difficult.
Finish plaster is used for the thin surface coat that is applied over the undercoat.
Always store plaster in a dry place. If any water comes into contact with plaster before it is used, the properties of the plaster will be altered. You should use plaster as soon as possible after buying, as the retarder (the constituent which governs the setting time) grows less effective with time. Plaster is usually date stamped on the bag and, whenever possible, you should use the plaster within six weeks of the date.
Q My newly applied plaster keeps cracking, I think because the old, porous plaster is drawing the water out of it. Is there a remedy ?
A Yes. Mix one part of PVA adhesive with four parts of water. Paint it onto the exposed brickwork and thoroughly soak the edges of the existing plaster. You should hear a 'fizzing' noise as the plaster soaks up the mixture; if not, the mixture is too rich. Apart from forming a water barrier, this treatment helps stiffen the old plaster.
