First Coat Scratch Coat

The first coat of plaster, the scratch coat, is the most time-consuming and material-intensive coat to apply. It is forced into the straw to achieve a good bond and built up to fill the hollow spots and irregularities of the bales. Plasterers who are experienced at covering straight walls are invariably surprised at the amount of time and material it takes to build the scratch coat on a bale wall.

The scratch coat uses the strongest plaster mix with a higher amount of binder to aggregate. Plaster is applied thickly enough to cover most of the straw ends, on average ^ to 1 inch thick, with most of this material being pushed into the straw. It is called the scratch coat because you will literally scratch the surface of the plaster with horizontal strokes once the plaster has begun to set. These scratches will afford the next coat a mechanical grip and supplement any chemical bonds that are formed.

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