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What is a Scratch Coat?

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  • Written By: Malcolm Tatum
  • Edited By: Bronwyn Harris
  • Last Modified Date: 30 August 2014
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Scratch coats are the bottom layer or coating of plaster that is applied to a wall. While still wet, this coat is scratched using a trowel or some type of serrated blade. The creation of a web of surface scratches helps the second coat of plaster to adhere to the scratch coat properly by allowing the overcoat of plaster to enter the scratches and create a bond between the two layers.

The application of scratch coating can be utilized with just about any type of wall, including new construction. The scratch coat can be applied to such diverse wall surfaces as stone, sheetrock and even brick. Once this base coat is applied to the wall, it is allowed to dry slightly. However, the creation of the scratches takes place before the first layer of plaster has a chance to set. This leaves behind a rough surface with tiny indentations that can be filled as the second coat of plaster is administered.

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While there are comb-like tools that can be used to make the scratches in the underlying coat, it is more common to make use of a trowel or even a serrated knife to create the series of scratches. The actual task of making scratches in the plastering is not difficult at all. Since the scratch coat is the first layer of plaster on the wall, there is no need to spend a lot of time smoothing the surface before initiating the scratching process. Essentially, the scratch coat simply needs to be relatively smooth, but with small scratches distributed all along the surface of the wall.

Once the scratch coat has been allowed time to set, the finish coat can be applied. While plastering on this top coating, the second layer of plaster will fill in the scratches on the underlying coat. This action essentially creates points all along the surface of the wall where the two layers are joined and are allowed to create a bond that provides greater stability to the finished plaster.

While it is possible to use this combination of a scratch coat and a finishing 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 collect in the tiny niches in the brick surface or even leech into the brick from the ground or the roofline along the top of the wall. The salt will delay the drying process of the plaster significantly.

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Perdido
Post 4

My friend paints houses for a living, and I once went to work with him and watched him apply stucco. He did a scratch coat, a brown, coat, and a finish coat of stucco.

He had to keep the scratch coat moist until it was time to apply the brown coat. The brown coat is just as thick and made of the same stuff as the scratch coat, but you don't score it at all.

The finish coat is thinner than the previous two. It is the final coat, and since you already have two thick coats beneath it, there is no need for it to be thick.

cloudel
Post 3

I have seen scratch coats of plaster that never got painted over, and they were very rough looking. A lot of buildings downtown in my area look old and abandoned, and a few of them just were never completely finished, yet businesses still operate inside of them.

One building is an art gallery. The scratch coat layer is all you can see once you step through the doors. If you touch it, it is rougher than the surface of brick!

I guess it's okay for an art gallery to have scratch coat walls, because it's a bit unconventional. Also, the focus is meant to be on the art itself and not the building.

shell4life
Post 2

@DylanB – It's best to score your stucco scratch coat all in one direction. Don't score it vertically, because this can cause the water you will be spraying on it to keep it moist to run off too quickly. Also, it could cause cracking.

So, scratch the coat horizontally. You don't need to do any diagonal scratches or mix it up, because that could actually weaken the layer.

I like using a rake made for scoring plaster. This is the easiest way to get a series of even horizontal lines.

DylanB
Post 1

I am going to be applying stucco to my walls soon as part of a remodeling project. I know next to nothing about the process of applying a scratch coat, and I'm hoping someone can help me out.

Does it matter if I make the scratches horizontally or vertically? Should I mix up the direction in which I make the scratches, or should they all go in the same direction?

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