What is the correct order of coats in Gypsum plaster installation?

• A. finish coat, brown coat, scratch coat
• B. scratch coat, brown coat, finish coat
• C. brown coat, finish coat, scratch coat
• D. base coat, finish coat, top coat

Answer: (B)

The order of coats is driven by how each layer bonds to the one beneath and how it prepares the surface for the next step. The first coat, the scratch coat, is applied to the substrate and then scratched to create a rough, keying surface. This key is essential for mechanical bonding, helping the next layer grab onto the base rather than sliding off.

The second layer, the brown coat, goes on over that key and adds the bulk and thickness needed to establish a stable, even plane. It fills in minor irregularities and starts building the wall out toward a uniform surface, while still allowing for some moisture movement during drying.

The final layer, the finish coat, is thin and designed to produce a smooth, ready-to-paint surface. Since it relies on the integrity of the two underlying coats, it must be applied last after the substrate has a solid key and adequate thickness built up.

Applying the finish coat first would leave no key or adequate base for bonding, and applying the finish before the brown coat would prevent proper thickness and leveling. So the sequence scratch coat, then brown coat, then finish coat best achieves adhesion, strength, and a smooth final surface in gypsum plaster installations.