Plastering - Application

On normal surfaces, such as brick- or blockwork, two-coat work (float and set) is common, with an undercoat 11mm thick and a finish coat 2mm thick. The undercoat should be lightly scratched to form a key, and the finish coat should be applied when the base coat is firm but not dry. On metal lathing two undercoats are usually necessary, the first filling the gaps between the lath and the second to provide even suction for the finish or setting coat.

On board finishes (see the next section on plasterboard and dry-lining) two-coat work can be used but, unless the boards are being plastered to improve thermal insulation or fire protection, there is no need for the floating coat. A setting or finish coat is quite adequate as long as the boards have been fixed correctly.

When mixing gypsum plaster it is important to use clean water and clean tools to avoid contamination, which may accelerate or retard the setting time. To ensure good adhesion and to exclude any pockets of air the plaster should be applied with very firm pressure. Trying to put thick coats on in one layer is therefore rarely successful.