Cement stucco is a popular material used for siding on the exterior walls of a commercial building or home. It provides a light-weight yet durable finish and can be easily troweled on the wall surface and patterned if so desired. Several coatings are applied in layers to arrive at the needed application. The base coat consists of three layers and those are the dash coat, the scratch coat, and the brown coat. Applying these different coats correctly will determine the success of the entire project.
Step 1 - Prepare
If applying cement stucco to masonry, make sure to clean the surface area from any and all loose particles. A clean surface area free of dirt and grease that is sufficiently “roughed up” is necessary for the stucco coatings to adhere properly. If the surface area where stucco is to be applied is not sufficiently rough, as may be the case with wood, apply metal reinforcing before applying stucco. Since stucco is made from the same ingredients as concrete, it will bond well when applied directly. The use of a water-based emulsion will act as an additional bonding component.
Step 2 - Install Reinforcement Materials
Embedded materials such as metal mesh, netting, lath or wire fabric will strengthen the stucco coat, allowing proper application on unstable surfaces. Make sure any and all metal elements are galvanized or at least rust-resistant coated. On sheathed walls, a waterproof paper has to be applied before metal lathe is installed over it.
Step 3 - Apply the Base Coat Layers
The typical application of cement stucco calls for layering four coats: three base coats that are known as the dash coat, the scratch coat, the brown coat, and the finish coat. Each base coat has a particular mix:
Dash Coat: Mix one part cement and two parts sand with sufficient water to spatter a coat across the entire surface area.
Scratch Coat: Mix one part cement, ¼-part lime (hydrated) and 3-to-5 parts sand with sufficient water to make a workable mixture. The scratch coat is applied in a ½-inch thick layer completely embedding the metal lath. Make sure to trowel groove the surface area so the next base coat layer, the brown coat, can bond successfully. With the metal lath embedded, the scratch coat creates a perfect base for applying the brown coat after drying for a few hours.
Brown Coat: Has the same mixtures as the scratch coat. The difference between the two is in the application. The brown coat should be troweled on evenly to a 3/8-inch thickness presenting a smooth surface to receive the finish coat.
Make sure to slightly dampen each coat before applying the next one to aid in application and bonding. Allow several days of drying time before applying the finish coat.