Interior plasters and exterior renders create seamless, continuous coatings on the surface of walls and ceilings. The purpose of these wet-applied coatings is to protect the structures, create smooth finishes, and serve as a base for paints or various claddings. These mortar-like coatings typically use lime, cement, or gypsum as binders and can be mixed entirely on-site or prepared from pre-manufactured dry mixes.
1. Plaster
The typical thickness of interior plaster is 1 to 1.5 centimetres. After skim coating, the surface is typically painted or wallpapered. The plaster is usually applied in several layers:
-
First, a spatterdash surface is created, or a primer is applied.
-
Next, a gypsum or lime-cement base coat of plaster is applied with a thickness of 10–12 millimetres and a grain size of 0–4 millimetres.
-
Finally, a finer, 2–5 millimetre thick finishing coat is applied with a grain size of 0–1.5 millimetres.
For a lower standard of finish, or where the plaster will be covered by cladding, it is sufficient to apply the plaster in a single layer, completing the smoothing process on that one layer.
2. Render
Unlike plasters, render must also withstand weather conditions such as sunshine, wind, temperature fluctuations, precipitation, and frost. Rendering follows the same principles and steps as plastering, but in this case, the top coat is a much durable, coarser or textured finish. The finishing coat can be a traditional aggregate render, a thick or thin-coat decorative render, or a silicate, silicone resin, or synthetic resin-based thin-coat render.
a) Thin-coat render
Nowadays, the most common solution is a thin-layer render, whose finishing coat with a maximum thickness of 5 millimetres, is simultaneously weather-resistant, water-repellent, washable, and vapour-permeable. To apply the thoroughly mixed primer of the thin-layer render, a temperature above 10 °C is required, and the application is carried out with a brush or paint roller. The rendering mortar is a factory-prepared wet mixture which when it has been thoroughly stirred, can be applied 24 hours after the primer has dried. The building facade should be divided into sections so that each surface can be plastered in a single, uninterrupted operation. Application is done using a trowel or plastic float, working from top to bottom. The finished surface can be textured with a dragged or scraped pattern by mixing the aggregate into the render using a hard plastic or wooden float.
b) Dryvit render
A special type of render called the dryvit render is applied to walls with façade insulation. Thermal insulation made of expanded polystyrene or rock wool is mechanically fixed directly onto the raw surface of the wall structure using adhesive and dowels. A smoothing layer is created on the insulation surface with adhesive mortar, into which a reinforcing mesh is embedded. A smoothed surface is formed using a trowel. The primer of the thin-layer render and then the render itself are applied onto this surface, as described earlier.
c) Plinth render
Plinths require special protection, for which resin-based thin-coat renders are excellent. These renders contain coloured aggregates to provide greater resistance. If thermal insulation is needed, waterproof extruded polystyrene insulation is used beneath the render, with the thin-coat render applied on top of it. Above the plinth, a drip edge aluminium profile must be installed, and waterproofing sheets must be fixed either behind or above the plinth insulation.
