The scratch coat is the first coat of stucco applied to the wall. Its name comes from the fact that it’s scratched horizontally to promote both a mechanical and chemical bond between it and the brown coat, which is applied next. Prepare the scratch coat according to the manufacturer’s instructions. Ensure that the proper proportions of cement, stucco-grade sand, and water are used in the mix. It’s particularly important to have the correct amount of sand, as too much sand can weaken the system and too little can increase the likelihood that it will crack. Let the mix sit for 20 minutes before remixing it for application.
Apply the scratch coat to the wall at a uniform thickness of 3/8-inch to a maximum of 1/2-inch. Make sure the scratch coat completely encases the metal lath. After it’s applied, scratch the coat horizontally. Scratching promotes a good mechanical bond between the scratch and brown coats and helps the scratch coat retain water during hydration.
Wait for the scratch coat to set up to 10 to 12 hours, depending on the air temperature and humidity, before hydrating the scratch coat. Hotter, drier climates will require that the hydration process begin earlier. To hydrate the scratch coat, flood the wall surface with a gentle stream of water, working from the bottom to the top until the water runs off in sheets. Applying water from top to bottom will cause the water to run down in rivulets, potentially eroding the stucco. Once the water has been absorbed by the surface of the scratch coat, it’s ready for another cycle of hydration. Hydrate each exterior wall at least once, but up to as many as three times, if the wall continues to soak up water. The stucco should retain moisture for the first 48 to 72 hours after it’s applied in order to gain the strength needed to fully support the brown coat.
