A professional plasterer can achieve a smooth, flawless finish in a relatively short time and with seemingly little effort. But the skill needed to do this takes years to master, and to achieve the same result the less experienced plasterer needs to spend some time in mastering the basics before starting important work.

As with other surface coverings the key to a successful finish lies in pre­paring the surface beforehand. On most surfaces two coats of plaster are applied, the floating coat followed by the finishing or setting coat. A perfectly flat final surface can only be achieved by preparing the underlying building materials properly and setting out the wall before plastering.

[edit] Preparing The Base

Plaster can be applied to any solid surface such as bare brick, cement rendering or building blocks and to surfaces which are relatively smooth (such as concrete and metal) providing they are adequately prepared. As well as checking that these materials are sound and dry it is often necessary to roughen them to provide a key for the plaster coating.

[edit] Brick And Building Blocks

These usually provide an excellent base for plastering since their rough exterior surfaces allow the plaster to adhere well. However many of these materials become badly damaged over time and efforts should be made to patch up cracks and to replace broken bricks before plastering is commenced. Crumbling and and decaying mortar joints should be cleaned out with a wire brush or cold chisel and re-pointed with fresh mortar.

[edit] Blown Plaster

When dealing with walls that have already been plastered previously, it is essential to carry out extensive investigations to check whether sections of the plaster have "blown" or separated from the wall beneath. Plastering on top of these would be a waste of time since the new plaster would increase the weight on the wall and result in pieces breaking away and unsightly holes appearing.

To test for blown plaster tap the wall at intervals with the handle of a hammer, a hollow sound indicates the material is loose. Hack away all the affected plaster using a hammer and wide bolster until you get back to a sound base.

If you discover that only small areas are loose these can easily be repaired by patching. If the whole wall or large parts of it are blown then you have no choice but to remove all the loose and damaged material before re-plastering.

When carrying out these repairs it is worth considering why the plaster has worked its way loose. Sometimes it is a case of poor adhesion if the wall was inadequately prepared when the work was first done. But blown plaster is more usually a sign of penetrating damp and if you suspect that this is the case a cure must be found and the wall surface sealed before you begin plastering.

[edit] Plasterwork

Some surfaces such as plaster are usually too smooth to provide a good key for re-plastering and must be roughened in some way to aid adhesion.

To do this, mark a grid of criss-cross lines on the material carefully using a heavy hammer and wide bolster. Wear safety glasses when doing this to protect your eyes from flying masonry. The lines should be about 50mm apart and need not do more than break the surface to be effective. Softer materials such as plastered over plasterboard can be scored by drawing the blade of a bricklayer's trowel across the surface.

To increase adhesion even further use a PVA bonding adhesive on the surface before plastering. These are available either ready mixed or in powder form and should be used according to the maker's instructions.

[edit] Types Of Plaster

Traditional plasters were based on lime and sand mixed with cement but these have largely been superseded by lightweight plasters which are based on gypsum. They are ideal if you have not done any plastering before since they need only the addition of water to make them workable. As a guide to quantity, when using this type of plaster, an area 600mm square needs about 3kg of coarse plaster and 1.3kg of finishing plaster.