Plastering - Difficult backgrounds

Stable backgrounds with a good key and moderate suction are suitable for most types of gypsum plaster. Materials with high suction, such as aerated blocks, can present a problem. Special plasters with water-retaining agents are often recommended because aerated blocks absorb water readily. Water in the mix may be absorbed to such an extent that the chemical reaction which binds the plaster together cannot occur. Some plasterers try to get round this problem by wetting the blocks prior to plastering, not always with successful results. It is difficult to wet the blocks consistently and very wet blocks will expand slightly and shrink on drying. This can lead to cracking. Success can be achieved by spraying the wall sparingly with a fine ‘mist’ of water.

Some very smooth surfaces, such as dense in-situ concrete, offer a poor key and low suction. In this situation it is common practice to specify a PVA adhesive to be applied to the background before the application of the undercoat or single coat plaster.

Some materials, such as timber or very porous old stonework, are unsuitable for plastering directly. In such cases a galvanised or stainless steel lath can be fixed to the background and this acts as a mechanical key for the plaster. A good example of this would be where the masonry wall meets the timber wall-plate which supports the roof structure.

Large areas of metal lath should be plastered with a plaster designed for such application. When plastering metal lath an initial coat, known as the ‘pricking-up’ coat is required. This should be forced through the lath and lightly scratched to form a key for the floating coat. There are a variety of metal laths on the market.