Scratch coatings are the first layer of plaster on a wall that are scratched while wet to create a network of superficial scratches. This allows the second coat of plaster to adhere correctly and create a bond between the two layers. The scratch resistant coating can be used on any type of wall surface and is applied before the first layer of plaster sets. Once the scratch layer has hardened, the top coat can be applied, filling in the scratches and creating a bond for greater stability. However, using this approach on brick walls is discouraged due to salt build-up that can delay the drying process.
Scratch coatings are the undercoat or coating of plaster applied to a wall. While still wet, this coat is scratched using a trowel or some type of serrated blade. The creation of a network of superficial scratches helps the second coat of plaster to adhere correctly to the rendering, allowing the plaster overlay to enter the scratches and create a bond between the two layers.
The scratch resistant coating application can be used with virtually any type of wall, including new construction. The scratch resistant coating can be applied to such different wall surfaces as stone, drywall and even brick. Once this base coat has been applied to the wall, it is allowed to dry slightly. However, the creation of the scratches occurs before the first layer of plaster has a chance to set. This leaves a rough surface with small indentations that can be filled in while the second coat of plaster is being applied.
While there are comb-like tools that can be used to make scratches in the coat underneath, it’s more common to use a trowel or even a serrated knife to create the series of scratches. The actual task of creating scratches in the plaster isn’t difficult at all. Since the scratch coating is the first layer of plaster on the wall, you don’t need to spend a lot of time sanding the surface before starting the scratching process. Essentially, the scratch coating simply needs to be relatively smooth, but with small scratches distributed across the surface of the wall.
Once the scratch layer has had time to harden, the top coat can be applied. While plastering this top coat, the second coat of plaster will fill in the scratches on the coat underneath. This action essentially creates points along the entire surface of the wall where the two layers join and allow for the creation of a bond which gives greater stability to the finished plaster.
While you can use this combination of a scratch and finish coat to plaster any type of wall, many professionals tend to discourage using this approach on brick walls. The reason for this is that the small amounts of salt can over time build up in the tiny niches in the brick surface or even leach into the brick from the ground or roofline along the top of the wall. The salt will significantly delay the drying process of the plaster.
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