What is the purpose of applying a scratch coat?

• A. To provide a final smooth surface
• B. To key the surface for subsequent coats and create a good bond with the next coats
• C. To dry the wall quickly
• D. To seal the surface from moisture

Answer: (B)

The purpose of a scratch coat is to create a key for adhesion with the next coats. It’s the first plaster layer applied to the lath or substrate, and you scratch or comb the surface to develop shallow grooves. Those grooves, or key marks, give the next coat something to grip onto, forming a mechanical bond that helps the brown coat and finish coat adhere properly and stay in place. The scratch coat also helps smooth out minor irregularities in the substrate and build up the right thickness.

This coat isn’t meant to be the final smooth surface—that job belongs to the finish coat. It also isn’t about drying time or sealing against moisture; those are handled by later steps or different products. If you scratch too lightly, bonding can be weak; if you scratch too aggressively or apply it at the wrong time, you can create problems with cracking or poor bond.