Gypsum plasters

Most plasters in common use are produced from ground gypsum rock by a process that removes most of the moisture from the rock and results in a powder that sets hard when mixed with water. Setting times are controlled by the use of retarding additives which give each of the several types of plaster a setting time suitable to its purpose.

Gypsum plasters are intended for interior work only; they should not be used on permanently damp walls. They must not be remixed with water once they start to set.

Plaster of Paris

This quick-setting non-retarded gypsum plaster gives off heat as it sets. It is white or pinkish, and is mixed to a creamy consistency with clean water. It is unsuitable for general plastering, but good for casting, and can be used for repairs to decorative mouldings.

Carlite plaster

Carlite refers to a range of retarded gypsum plasters which are premixed with a lightweight aggregate and only need water adding to prepare them for use. The undercoat bonds well to most backgrounds, and this, coupled with their light weight — about half that of plasters mixed with sand — makes Carlite plasters fairly easy to use. The lightweight aggregate also gives some degree of thermal insulation. The average setting time for Carlite plasters is about two hours.

Four types of Carlite undercoat plasters — ‘browning’, ‘browning HSB’, ‘bonding’ and ‘metal-lathing’ — are available, each formulated to suit a background of a particular surface texture and suction. Browning is generally used for solid backgrounds with average suction (such as brickwork), while the higher impact- resistant HSB type is for high-suction backgrounds. Bonding undercoat is best for low-suction surfaces like dense brick or concrete blocks. Metal-lathing plaster is less commonly used and is primarily for an expanded-metal background.

When more than one undercoat layer is required to build up a thickness, the same plaster should be used for all layers to ensure compatibility.

There is only one Carlite finishing plaster; it can be used over all the undercoats, being applied as soon as the undercoat has set.

Thistle plasters

Thistle is the brand name of a range of building plasters used for a variety of conditions and backgrounds.

‘Hardwall’ is an undercoat plaster which provides superior impact and efflorescence resistance. It is suitable for most backgrounds.

There are two types of finishing plaster, both mixed with water only: multi-finish plaster, for use over sanded and hardwall undercoats, and board- finish plaster, used for finishing plasterboard surfaces.

Two special ‘renovating’ plasters are used on walls with residual dampness. The undercoat is a premixed gypsum plaster with special additives, and the finish plaster, formulated specially for use with the undercoat, contains a fungicide. These plasters are for damp walls that are slow to dry out, such as new exposed building work or in old houses where new damp-proof courses have been installed. This type of plaster is not itself a damp-proofing material, but it does allow the background material to breathe and dry out without letting the moisture show on the surface. Deal with the cause before you apply the plaster.

Sanded plasters

Before the advent of modern gypsums, lime and sand for undercoats and neat lime for finishes were employed in traditional wet plastering, often with animal hair added to the undercoat mix to act as a binder. Lime plasters are generally less strong than gypsum and cement-based plasters.

Lime is still used, but mainly as an additive to improve the workability of a sand-and-cement plaster or rendering. Cement-based sanded-plaster undercoats may be required by some authorities for kitchen and bathroom walls constructed on timber and expanded-metal lathing. These undercoats can also be used on old brickwork or where a strong impact- resistant covering is required.

Single-coat plasters

A universal one-coat plaster can, as its name implies, be used in a single application on a variety of backgrounds, and trowelled to a normal finish. The plaster is sold in 40kg (881b) bags and only water need be added to prepare it for use. It will stay workable for up to an hour and some types can be built up to a thickness of 50mm 12in) in one coat.

One-coat plaster is also available in small packs, either ready-mixed or contained in mixing tubs. These are ideal for small repairs. For larger areas it is more economical to buy bigger bags and mix the plaster on a board in the usual way.

Ready-mixed plasters

A brush-on skim plaster is also available. It is applied up to 3mm (’/ain) thick with a wide brush, and smoothed with a spreader or trowel. When firm it is polished with a damp sponge.

Fillers

Fillers are fine plaster powders used for repairs. Some, reinforced with cellulose resin, are sold in small packs and need only mixing with clean water for use. They are non-shrinking, adhere well and are ideal for filling cracks and holes in plaster and wood.

Plastering is carried out using modern gypsum plasters or mixes based on cement, lime and sand. By varying the process and introducing additives, a range of plasters can be produced within a given type to suit different background materials.

Plasters are basically produced in two grades — one as a base or ‘floating’ coat, the other for finishing coats. Base-coat gypsum plasters are premixed types which contain lightweight aggregates. Base-coat sanded plasters that are based on cement or cement/lime have to be mixed on site with a suitable grade of clean, sharp sand. Fine-powdered finish plasters only require the addition of water before they are ready to use.

The following information deals only with those materials that are suitable for domestic work.