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 scratch coat is the foundation. It is the first layer of cement that embeds the wire lath and provides the mechanical key for subsequent layers. If the mix ratio is wrong, or the application is too thin, the entire wall system will fail under the stress of Southern California's seismic movement and thermal cycles.
This guide moves beyond "handyman recipes" to explain the ASTM C926 standards for mixing and applying a structural scratch coat.
1. The Golden Ratio: 1 to 3
Stucco is not concrete. Concrete is strong but brittle; stucco must be strong but breathable. The mix ratio determines this balance.
1 Part Plastic Cement : 3 Parts Plaster Sand
- Plastic Cement: We use "Plastic" cement (ASTM C1328) because it comes pre-mixed with lime, which improves workability and flexibility. Avoid using raw Portland cement unless you are manually adding lime.
- Plaster Sand: Use washed, angular plaster sand (ASTM C144). [Image of sand grain size comparison] Do not use river sand (too round) or play sand (too fine). The angular grains lock together for strength.
2. Mixing Consistency: The "Pudding" Test
The water-to-cement ratio is critical. Too dry, and it won't bond to the lath. Too wet, and it will slump off the wall (and crack as it shrinks).
The Goal: You want a "stiff pudding" consistency.
The Test: Load the stucco onto your trowel and hold it at a 40-degree angle. It should stick to the metal without sliding off, yet still be spreadable.
3. Material Calculation
Don't get caught short mid-wall. Stopping a scratch coat in the middle creates a "cold joint" that is a future crack point.
The Math:
1 Bag of Plastic Cement (94 lbs) + 300 lbs of Sand = Approx 50 sq. ft. of wall area (at 3/8" thickness).
Tip: Always calculate 10% extra for waste and dropping.
4. Application: Embed the Lath
The purpose of the scratch coat is to encase the wire mesh completely.
Thickness: Apply a minimum of 3/8" to 1/2" thick.
Technique: Press hard. You must force the mud through the holes in the wire lath so it keys into the paper behind it. If you just skim the surface, the wire will rust and the stucco will delaminate.
5. The "Scratch" in Scratch Coat
Why is it called a scratch coat? Because of the texture we leave behind.
Before the cement hardens (while still "green"), use a Scarifier (scratch tool) to cut horizontal grooves into the surface.
The Why: These grooves create a mechanical bond for the next layer (the Brown Coat). Without these ridges, the heavy brown coat may slide off the wall.
Once the scratch coat is applied, you must keep it moist. Mist the wall with a garden hose twice a day for at least 48 hours. This Hydration process prevents the cement from drying too fast and cracking. Do not apply the brown coat until the scratch coat has cured for at least 48 hours (ASTM standard).
Conclusion: Respect the Base
The scratch coat is buried forever, but it does the heavy lifting. Mixing it correctly ensures your home is fire-resistant, impact-resistant, and watertight. Don't cut corners on the sand or the cure time.
Last week, we shared How To Install 3-Coat Stucco Wire. You can't apply a scratch coat until the lath is perfect.