Generally speaking, plaster is a compound building material. It is not load bearing and used only for decoration or repair. There are many types of plaster defined by the compounding ingredients and application techniques. Plastering is a very old form of wall decoration, perfected with time and with the introduction of acrylic, metallic and other additives that allow to achieve many more visual qualities.
Today we will take a look at various types of plaster according to its content and intended usage.
Three main classes by content are lime, gypsum and cement plaster. Lime plaster is made of calcium hydroxide, sand and water. It was widely used until the early 20th century, while today is mostly reserved for repair of lime plastered surfaces in historic homes. Another use of this type of plaster today is in “green” construction. It is caustic while wet and dries slowly. The resulting finish is quite smooth due to the usage of very fine sand.
Gypsum and cement are main plasters used in construction and decorating today. Cement plasters are used mainly on the exterior walls and are known as stucco. We will have a detailed look at stucco in the next articles. Gypsum is the interior application type of plaster and is made with heated calcium sulfate. It is very suitable for applications over the gypsum board (plasterboard, or drywall) and dries quickly. Gypsum plaster if not water resistant on its own, is relatively soft and can be textured. A wide range of looks and surfaces can be achieved with this type of plaster, especially with the use of different additives for elasticity, color, and texture.
As far as their intended use plasters are divided into PVA bonding, backing browning and bonding plaster, and finishing/skimming plaster. PVA, strictly speaking, is not a plaster. It is well-known glue. However, we mention it here because it is an essential first component of a plastering process. PVA corrects the lack of water resistance by creating the insulating layer. It also helps prevent premature drying and cracking of the plaster. And even more, it helps the plaster stick to the wall surface better.
Browning or bonding plasters are backing plasters. Browning if intended for porous, absorbent materials like brick and building block. It is applied in a thick coat of about 10mm and sets in about 2 hours. Bonding is applied to non-so-porous materials like hard surface brick or block treated with PVA. It is applied a little thinner (about 8 mm) and also takes about 2 hour to set. Both backing and bonding are “wet” materials and can take several months to dry out completely and require skill to apply to an entire wall. Therefore, these materials are not usually used in new construction, and plasterboard (drywall) is a preferred backing for a plaster.
Finishing, or skimming, plaster is applied as a last coat of about 2mm. It can be applied over backing plaster or directly over plasterboard. Drying time of this plaster depends on the room conditions. Finishing plaster needs to be worked with a trowel when it just starts to set.
One coat plasters are available and are intended to combine all the above stages together. However, plastering contractors do not favor them and prefer to follow the traditional routine for the best, professional results.
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More on the plastering and plasters to follow. Thanks for reading.