Stucco

Stucco first appeared thousands of years ago as a mud plaster baked by the sun. After several millennia, it came to be a mixture of gypsum and lime. Stucco has continued to improve, keeping pace with consumer styles and technological improvements. Its purpose, however, has remained the same.

Stucco is a finish application of three relatively thin coats of mortar designed to protect and beautify a surface. It's a good example of the design flexibility cement-based products offer.

Stucco can transform the appearance of a block wall -- or any wall. Because it's flexible before it cures, you can mold and shape it to complement any style.

The first coat of stucco, about 1/2 inch thick, is called the scratch coat. That's because it's scratched with a scarifier or other tool to roughen the surface, thereby forming ridges for the second coat to bond into. The second coat is about 1/4 to 3/8 inch thick and is called the brown coat. The third or finish coat is about 1/8 to 1/4 inch thick and is sometimes called the color coat because it is often tinted with oxide pigments.

Although you can paint stucco, it is unwise to do so because it's difficult to repair damage to painted stucco. The mortar won't stick to a painted surface even with a bonding agent.

Stucco's application to a block wall requires little preparation. Simply mist the wall lightly with water; dry block would take moisture too rapidly from the stucco. Wood surfaces require an application of felt paper and metal lath to give the surface tooth for the stucco. Lath comes in three forms: paper-backed lath, with the paper already attached for quicker installation; self-furring lath, with a dimpled surface that holds it away from the wall; and galvanized metal lath, for use on exterior walls. You can install corner beads to strengthen the corners and prevent stucco from chipping.

In any case, when you're ready to start the job, wait for an overcast day if you can, especially when stuccoing walls that have a southern exposure. Excessive heat dries the stucco too quickly, causing it to shrink and crack. Cool temperatures, on the other hand, stiffen the stucco, preventing proper troweling. Do not apply stucco if nighttime temperatures will fall below freezing. The ideal temperature for installing stucco is between 50 and 80 degrees Fahrenheit.

Stuccoing is a slow-moving job. Although you will soon develop a knack for applying the stucco, this material, like all cement-based products, needs adequate time to cure properly. If codes allow, you can apply the brown coat to the scratch coat after the first coat has begun to set. Both coats must cure thoroughly before you apply the finish coat. Otherwise the stucco may crack.

Designing with stucco

The color range for stucco is somewhat narrow. You'll find white, browns, greens, reds, and blues, generally in muted shades. Texture varies depending on the tools you use to embellish the finish coat. Possibilities include whisk brooms, brushes, and natural objects such as leaves and branches. You can also use dowels or other objects as styluses to scratch designs into the surface.

One-coat stucco

A newer material is the one-coat stucco. Ingredients are proprietary to each manufacturer, but most include fibers and other performance enhancers that help resist cracking. One-coat stucco is intended as a base-coat application with a finish coat applied after curing. The material is available both in concentrated form, requiring the addition of sand and water, and in presanded versions requiring only the addition of water.

One-coat stuccos tend to go on faster, weigh less, and are more flexible than three-coat stuccos. While they are not as thick as three-coat stuccos, they are strong and durable.

Finish-coat products made with fibers in an acrylic solution are also available. These acrylic-base finishes are strong and flexible, and come in a wide range of colors that, because of the nature of the acrylic base, tend to be slightly more vibrant than the muted tones of the oxide pigments.

Pro Tip: Making stucco

You can buy stucco at home centers in premixed bags. Manufacturers achieve a much higher degree of accuracy when mixing proportions than you can with the shovel-and-mixer method. On the other hand, masons have been mixing stucco from scratch for years, and you can too. The basic ingredients are portland cement, lime, and sand. The only difference between coats is the proportion of sand.

Scratch coat
1 part portland or masonry cement
1 part hydrated lime
2-1/2 to 4 parts sand

Brown coat
(for brick walls)
1 part portland or masonry cement
1 part hydrated lime
3-1/2 to 5 parts sand (1 part more than scratch coat)

Finish coat
1 part portland or masonry cement
1 part hydrated lime
1-1/2 to 3 parts sand (1 part less than scratch coat)

 

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