What is the minimum thickness of the scratch coat?

What is the minimum thickness of the scratch coat?

• A. 1/4 inch
• B. 1/2 inch
• C. 3/8 inch
• D. 1/8 inch

Answer: (C)

The main idea is that the scratch coat must be thick enough to form a solid base and to provide a good key for the next layer. That requires enough material to hold up during subsequent coat applications and to create a rough surface for bonding. The minimum thickness that achieves this is 3/8 inch. If it’s thinner—like 1/8 or 1/4 inch—it won’t give a reliable key or enough strength, leading to bond failures or cracking of the overlying coats. A thickness of 1/2 inch can be used in some situations, but the question asks for the minimum, which is 3/8 inch.

The main idea is that the scratch coat must be thick enough to form a solid base and to provide a good key for the next layer. That requires enough material to hold up during subsequent coat applications and to create a rough surface for bonding. The minimum thickness that achieves this is 3/8 inch. If it’s thinner—like 1/8 or 1/4 inch—it won’t give a reliable key or enough strength, leading to bond failures or cracking of the overlying coats. A thickness of 1/2 inch can be used in some situations, but the question asks for the minimum, which is 3/8 inch.