Gypsum Plaster and Skim Coat

Gypsum and soil plaster is a mix of building gypsum with sieved clay soil, typically at ratios between 1:1 and 1:2 by volume. The soil extends the setting time enough to allow screeding and reduces cost. The coat is applied 15 to 25 mm thick, and its main job is to bring the wall flat and plumb. A finishing skim of pure gypsum, 2 to 5 mm thick, is then applied to produce the smooth surface ready for paint.

Building gypsum has an initial set of roughly 8 to 15 minutes, and that short window makes the work entirely dependent on the crew's speed and coordination. Gypsum re-mixed with water after setting has begun never regains its original strength. This practice, common on site, is the main cause of skim coats blistering and delaminating later.

Another frequent error is plastering over dusty substrates or dry AAC blocks. The surface must be clean, dampened and, where needed, primed. Plastering directly over metal without lath or reinforcing mesh produces a straight crack along the contact line, and the same happens where electrical conduits and services run. The fix is fitting mesh or reinforcing tape over service routes before plastering.

Gypsum does not survive permanently damp conditions. For bathrooms, the wall behind kitchen sinks and any surface exposed to moisture, cement render or moisture-resistant board is the correct choice. Final acceptance is checked with a 2 m straightedge; a deviation of more than 3 mm will show through the finished paint.