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Applying Stucco

Stucco makes an excellent exterior surface. It is durable and weather-resistant, has a pleasing texture, and can be painted. Even in severe climates, you’ll find older stucco houses still in mint condition. A stucco finish is nothing more than two or three thin coats of a mortar that is 1 part masonry cement to 3 parts sand, with a small amount of lime and water added. Stucco requires a solid backing. Never apply stucco over fiberboard sheathing or foam insulation. Both of these materials give enough that a well-thrown baseball can dent or puncture the stucco wall.

There are infinite possibilities for the final textures. The examples shown above right represent some of the standard textures, but you can experiment with different trowels and techniques to create different textures. Just be sure you can reproduce the texture consistently over a broad area. If left untinted, stucco dries to a medium-gray color. You can add an oxide pigment to the finish coat or stain or paint the surface after the top coat has cured. If you mix in pigment, carefully measure and mix each batch exactly the same way to obtain a consistent color. You can make a bright white stucco by mixing together white Portland cement, lime, and white silica sand for the finish coat.

Tools: Brush, hammer, tin snips, flat trowel, hock, plasterer’s rake or homemade scratcher, hose. (For scratched finish texture, a brush or whisk broom.)

Select a texture. The wide range of finish textures available makes stucco a versatile wall covering. To achieve a smooth, plasterlike appearance, trowel the final coat several times as it becomes progressively stiffen For a swirled texture, trowel the mortar just once, using an arcing motion, and allow the resulting pattern to remain. For a wavy, scratched surface, trowel the mortar smooth, allow it to harden slightly, then draw a brush across it lightly. The stiffer the brush, the coarser the pattern. To stipple the top coat, hold a whisk broom at an angle to the wall and pat the surface with the ends of the bristles in an irregular pattern. For a travertine finish, spatter on a coat of thin mortar in a contrasting color and trowel it slightly after it has stiffened. To make an imprint, use leaves or other patterns to make imprints in the soft mortar and trowel the surface lightly.

1. Prepare wall, apply first coat. For a concrete, brick, or block wall, simply brush on concrete bonding agent and allow it to dry. Apply stucco directly to the wall. To apply stucco over a wood wall, nail on 15-pound roofing felt, then cover it with 17-gauge metal netting (buy 150-foot rolls to minimize seams). Cut the netting with tin snips and attach it with galvanized roofing nails. Apply the scratch coat with a flat finishing trowel. Trowel on a 1/4 to 1/2 inch layer of mortar, forcing it into the netting so some extrudes through the netting to “key” the coating in place.

2. Scratch the first coat. Once you start one wall, always complete it to avoid start-and-stop lines. Allow the scratch coat to harden only slightly, then scratch it with a plasterer’s rake or a homemade tool like the one shown. (To make it, simply drive 4d galvanized nails through a piece of 2x2 at 1-inch intervals.) Scratch the entire mortar surface to a depth of about 1/8 inch, running the tool in long lines along the surface.

3. Keep the mortar wet. As with all concrete or mortar products, slow, damp curing provides the greatest strength. Allow the scratch coat to cure for 36 to 48 hours; keep it damp by periodically misting it with water from a garden hose. Watch the weather; you’ll need to mist more often on a hot, dry day than on a cool, damp day.

The Brown Coat

■ For an extra-strong stucco wall, you can apply a coat of mortar between the scratch and the finish coats. This is called the brown coat.

■ Apply the brown coat soon after you scratch the scratch coat. If you must wait before applying the brown coat, keep the scratch coat wet after scratching it to keep it from curing. Mix, apply, and scratch the brown coat in the same way as you did the scratch coat. Allow it to set for a few hours, then keep it moist for two days for a slow cure.

4. Apply the finish coat.

4. Apply the finish coat. With a flat finishing trowel, apply a 1/8- to 1/4 inch thick finish coat onto the dampened scratch or brown coat (see box at left). If you add powdered pigment, add water to the pigment and mix it completely before adding it to the stucco. Finish to the texture of your choice. Allow the stucco to cure for several days, misting the surface occasionally to slow the curing process. Complete the project by caulking around doors and windows. If you paint the stucco, wait at least six weeks before you paint and use a paint formulated to cover concrete.

Using New Stucco Products

The stucco technique is the least expensive way to cover a wall with stucco, but it takes a long time. It can be especially tedious to keep the various coats moist for several days. New products and techniques are now available. The materials are more expensive, but installation is easier. Flashing and a water barrier are installed in such a way that water that comes into the wall (through small openings around windows, for example) can escape, keeping studs and insulation dry. Instead of a scratch coat of stucco, sheets of cement board form the substrate. This provides a straight, even surface, something not easy to achieve with standard stucco methods. The cement board is covered with a thin coat of Portland cement, then with a coat of aggregated polymer, which is applied without special troweling techniques. Available in many colors, the polymer surface resists dirt and cleans more easily than a standard stucco finish.

Tools: Hammer, chalk line, level, stapler, trowel, knife, drywall square.

Install a polymer stucco wall. Remove window and door moldings and trim out with filler blocks that are the same thickness as the new stucco wall. Along the bottom of the wall, use a level and chalk line to mark a line, then install flashing along the line. Staple 15-pound building felt (tar paper) to the wall, taking care not to rip it. Overlap joints about 3 inches. Install a metal starter track on the flashing. Attach the cement board sheets by slipping them into the starter track and fastening them with 2-inch roofing nails or screws designed for cement board. To cut cement board, use a drywall square and a utility knife. Cut into the board and through the mesh on one side, break back the board, and cut through the mesh on the other side. Fill in joints between cement boards with a mixture of Portland cement and water. Once that sets, cover the entire surface with a thin coat of the Portland cement mixture. After that layer dries, trowel on the polymer texture to a thickness of about 1/8 inch.

Caution! Beware of Insulation-Based Systems - Some polymer systems use sheets of soft insulating material, such as polystyrene, as the base for the polymer instead of cement board. Such systems have two serious problems: They trap moisture inside walls and dent easily.

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