Written by Stucco Champions — Southern California’s Authority on Exterior Plastering.
Scratch Coat Stucco Guide: Purpose, Mix, and Application Steps
In a traditional Three-Coat Stucco system, the finish gets all the glory, but the Scratch Coat does all the work. It is the foundation layer that embeds the wire lath and creates the mechanical bond for the rest of the wall. If the scratch coat is weak, too thin, or cured improperly, the entire stucco system will delaminate.
This guide breaks down the technical requirements for the first coat of stucco, based on ASTM C926 standards, ensuring your project starts with a rock-solid base.
1. What is the Scratch Coat?
The scratch coat is the first layer of cement applied to the wall. Its primary function is embedment. It must be pushed through the galvanized wire lath so that the cement completely encases the steel, protecting it from rust and locking it to the building paper.
Why "Scratch"? While the cement is still wet, we run a tool called a scarifier (a metal rake) horizontally across the surface. These grooves increase the surface area, providing a "mechanical key" or grip for the second layer (the Brown Coat) to bond to.
2. The Mix Ratio: Plasticity is Key
Stucco is not concrete. Concrete is rigid; stucco must be workable and breathable.
The Standard Mix (ASTM C926):
1 Part Plastic Cement to 3 Parts Sand (approximate).
- Plastic Cement: We use ASTM C1328 Plastic Cement, which comes factory-blended with lime. The lime adds "plasticity," making the mud sticky enough to hang on a vertical wall without sliding off.
- Sand: Use clean, washed plaster sand. It must be free of clay and salt. The sharp edges of the sand grains provide the structural strength.
3. Application Thickness
Thickness is critical. If it's too thin, the wire will rust. If it's too thick, it will slump.
The Target: A nominal thickness of 3/8 inch.
This is just enough material to cover the wire lath completely and allow for the scoring grooves without exposing the metal underneath.
4. The Curing Process: Hydration
This is where most failures occur. Cement cures by hydration—it needs water to harden.
The Rule: You must wait a minimum of 48 hours between the scratch coat and the brown coat.
During this time, the wall should be misted with water ("fogged") twice a day to slow down the drying process. If the scratch coat dries too fast (especially in our SoCal Santa Ana winds), it becomes brittle and weak.
You cannot apply a scratch coat until the lath is inspected. Ensure the wire mesh is tight (no "pillowing") and properly stapled into the studs. Loose wire requires more mud to fill, making the wall heavy and prone to sagging.
Conclusion: The Anchor of the System
Think of the scratch coat as the anchor. It connects the beauty of the finish to the strength of the frame. By using the correct 1:3 mix ratio, achieving full wire embedment, and respecting the 48-hour moist cure, Stucco Champions ensures the structural integrity of every wall we build.
Last week, we shared Stucco Scratch Coat vs. Brown Coat. Learn how this layer interacts with the leveling coat.