Plaster

Plaster is a mix based on lime cement or gypsum plaster, with or without the addition of aggregate hair or other materials, and water which is applied while plastic to walls and ceiling surfaces. It subsequently hardens to give a smooth surface for decoration. It may also be moulded into relief designs for use in ornate coving/cornices and ceiling centre pieces.

Some of the materials listed here may no longer be manufactured, but it is still worth mentioning them for the odd occasion that they may be encountered, particularly when refurbishing older buildings.

Acoustic plaster is plaster which has a higher level of sound adsorption. This is achieved by the addition of aluminium powder which when wet gives off a gas. This gas remains in the set plaster giving it a honeycombed structure. The sound absorbent properties of the plaster depends on its surface texture and porosity. Any surface coating is likely to affect the acoustic properties.

Barium plaster is retarded hemihydrate plaster that contains barytes aggregate with gypsum plaster. It is used in X-ray rooms in hospitals etc, to reduce the amount of radiation penetrating the walls.

Carlite is a range of lightweight retarded hemihydrate pre-mixed gypsum plasters, manufactured by British Gypsum. Carlite is produced in one finishing grade and four backing grades.

Fibrous plaster or stick and rag work as it is often called, is Plaster of Paris shapes made by casting the Plaster of Paris into gelatin or plaster moulds. The mix is reinforced with coarse hessian or canvas wire netting and wood lathes. The normal application for this type of work is ornate cornice or centre pieces for ceilings. The surface when dry is highly absorbent. A thinned coat of Dulux Trade Primer Sealer should be applied as a sealing coat when the surface is dry.

Keene's cement is an anhydrous gypsum plaster which is harder and less porous than the retarded hemihydrate type plasters. Keens cement can be trowelled to give a smooth glass-like surface. This can cause problems in terms of poor adhesion and "skidding" when painted. If this problem does arise one method that is quite successful is to apply a well thinned coat of Dulux Trade Alkali Resisting Primer as this will penetrate the surface and provide a key, whereas a thinned coat of emulsion paint will sit on the surface. However it is essential that the surface is dry in depth before using solvent-based primers.

Lime plaster is a plaster mix which has hydrated lime mixed in with clean sand. The surface is very highly alkaline. It is only used today in renovation work in old churches and listed buildings.

Textured plaster The most commonly used textured plaster is ARTEX. This is applied to plaster board ceilings. A variety of different patterns and texture can be achieved. The surface is not normally painted and if left unpainted for several years can become very powdery and friable. When this occurs it will be necessary to apply a coat of Dulux Trade Primer Sealer to stabilise the surface before painting.

Thistle is a range of plasters from British Gypsum. They are manufactured from gypsum rock which has been crushed and ground to a fine powder and then heated. Certain additives are included to produce the desired properties and setting time of the plaster.

Thistle Multi-Finish is a general purpose final coat plaster specially formulated for a wide variety of backgrounds including grey and decorative faced plasterboards. It has a special additive to control working and setting but should not be over trowelled. If over-trowelled and polished it should be treated in a similar way to Keene's cement (described above) in terms of painting.