APPLYING STUCCO
Applying Stucco

Introduction

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 link "Select a texture" in the Index above will show you some of the standard textures. But you can experiment with different trowels and techniques to find the texture that pleases you. Just make 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.

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 stiffer. 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 4-penny 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.

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 "experts insight" link). 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 (see Select a Texture). 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.

BHG

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