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Exterior Plastering: Many Faces of Stucco

Stucco is a type of plaster made by mixing cement, sand and lime with water and applying it to exterior and interior walls and sidings while still soft. It is also known as Portland Cement Plaster and is more common on the outside of homes. Properly applied and maintained, it can last for many years. It is strong, fire-resistant and requires very little maintenance. Stucco is breathable, making it rot- and fungus-resistant. This type of plaster is poorly suited to very smooth applications, but it does well when it includes texturing elements. Cement plaster takes dyes easily and is more likely to be colored than lime or gypsum plasters. Stucco can be made to resemble wood, brick or stone by using dyes, stamping techniques, or applying different material onto the wet stucco.

Traditional Stucco

The earliest forms of stucco used lime, sand, and water with the occasional addition of supporting fibers. Although the water soluble properties of lime allow this stucco to repair itself to some extent, it is easily broken or cracked. If this (or any other) stucco cracks, water between the plaster and the wall can cause the finish coat to fall off in chips and sheets.

Portland Cement Stucco

Modern stucco is made from Portland cement, sand, water and strengthening elements. Some lime may be added to improve workability. Portland cement hardens through a chemical reaction with water, rather than simply drying.

Three-Coat Stucco

Much like other types of interior plaster we looked at in the earlier articles, stucco is typically applied as a sequence of a “scratch” layer, a “brown” layer for a level body of stucco, and a final textured layer.
The three-coat method of applying traditional stucco enables the plasterer to create different finishes. A dash finish is achieved by flicking a trowel or brush over the surface. Sand float is created by adding more sand to a stiffer mixture and applying with a sponge. Scraping and using the side of the trowel will also create different finishes. Different stucco finishes give added dimension to walls. You can segment walls and provide highlight or depth to the wall finishes by color and texture.

Popular Finishes

- Smooth finish is achieved by smoothing the final layer several times using a steel float as the mortar begins to go off, occasionally plunging the float into water will keep it clean and prevent it pulling mortar away from the wall.
- Swirl texture is created by using a float in an arcing motion on the mortar just once while it is still fairly damp, and then leaving it to harden – a wooden float often gives better swirls than a steel float.
- Stippled texture. After the mortar has started to dry, a plasterer holds a stiff broom at an angle to the wall and pat the bristles onto the surface to give the desired pattern.
- Wavy, scratched texture. After smoothing the final layer, a plasterer allows it to dry slightly, then lightly draws a brush across the surface using a path to give the desired effect. The bristles need to be kept free from mortar build-up by knocking the brush head after each pass across the surface. The stiffer the brush, the coarser the pattern.
- Imprint finish is achieved by smooth the final layer and using objects to make the desired imprint(s) (e.g. leaves, hands, paws, special stamps, etc.). Sometimes, plasterers would leave the surface to dry a little more and then would soften the edges of imprints with a float or a brush.

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