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Three-Coat Stucco, continued

The Scratch Coat

The scratch coat is the first coat of stucco, and it serves as the foundation for the next two coats. It should be harder than the brown coat, which we achieve by using a richer mix -- about one part cement to two to four parts sand, or 21 to 24 shovels of sand per sack of cement. After the scratch coat is applied, the plasterer scores the surface with a special scarifier rake or a scratching trowel. This creates extra surface area and a better bond for the brown coat. In the first few hours after the scratch coat is installed, the cement is very fragile and should not be disturbed while it slowly gains strength. Because the scratch coat is thin, the moisture evaporates quickly. If it dries out too quickly, it will be weak as a result of incomplete hydration. To prevent this, you'll need to mist the surface with water. Verify the hardness of the scratch coat before applying the brown coat. To test for hardness, we drag a nail over the wall. If the nail does not dig in but leaves a white line, the stucco should be hard enough. If the nail does dig in, then the scratch coat is not hard enough and should be wetted so the cement can hydrate more thoroughly. The scratch coat should be no thinner than the brown coat, and firm enough to resist pressure from your hand when you lean against it. At least 95% of the lath should be covered by this coat. If there are any "dropouts," they should patched or filled that same day.

The Brown Coat

This is a leveling coat that provides the flat surface for the finished wall. The brown coat also adds strength and thickness, and is in large part what determines the quality of the finish. It's a little sandier, at one part cement to three to five parts sand, or about 25 to 30 shovels of sand per sack of cement. The increased sand helps reduce the number of shrinkage cracks. The application of the brown coat also reintroduces water into the scratch coat; because of the additional coverage, moisture will be better retained in the base coat, resulting in even more thorough hydration and a good, dense base. You should not apply the brown coat until the scratch coat is hard. Floating the brown coat makes for a denser product -- we use hard rubber floats. Wood floats will provide a straighter surface, but they are rarely used nowadays.

The Finish Coat

This is what provides the final texture and color. Premixed finish coat material usually works fine, except I would avoid dark colors, especially reds. Because stucco colors are mineral pigments, dark colors are prone to spottiness and, if not well blended, will not match the color sample. The only limits on the texture and appearance of the finish are the imagination and skill of the applicator. If you are new to using stucco and are planning to hire a sub, visit his previous jobs. Watch for uneven textures and appearance, discoloration, poor color matching, and irregular thickness.

Weather and Curing

Curing is an important part of a top-quality stucco job, and weather has a lot to do with curing. Weather conditions with high humidity, such as drizzly rain or heavy fog, are favorable to proper hydration of cement. If weather conditions are drier, you may need to gently spray the wall with a hose to ensure enough moisture for complete hydration and to avoid cracking. Stucco must be moist-cured for at least 48 hours, unless weather conditions are favorable (above roughly 80% RH, you don't need to moist-cure). According to code, there should be at least 48 hours of curing time after the first coat is applied, and seven days after the second coat is applied if there is to be a third coat. The critical consideration is that the scratch coat must be hard. As the first coat of stucco dries, it will lighten in color (Figure 6). This will be especially visible on the sunny side of the house first. When this happens you must fog or mist the stucco with water. Once it hardens (the next day), you don't have to be as careful -- you can squirt water on the wall.

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Figure 6. As the scratch coat dries it will change color. It is important to make sure there is enough moisture available for complete hydration of the cement by misting the wall with water.

You should also protect stucco from freezing for at least 24 hours after it has set; you also must not use frozen materials or apply materials to a frozen base. The bottom line is this: If the weather isn't right for stucco, you should wait until it is.

Site Protection

During application, part of the stucco will end up somewhere besides on the wall -- you can count on it. The responsibility for finish protection and cleanup should be carefully described in the scope of work. Make sure you know who is responsible for cleaning up the site and hauling away excess material and waste. Site protection, including cleanout areas, should be addressed before you start the job. Protection of glass, doors, window and door tracks, gutters, and landscaping from stucco slag is imperative (Figure 7) because, if left in place, finished surfaces can be irreparably damaged. For example, stucco can permanently etch glass in a matter of days. You'll want to clean up right away -- it's always easier to clean up the cement droppings before they're hardened.

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Figure 7. Finish protection is absolutely essential. Make sure you understand who is responsible for this before the job begins.

Cracks

Small shrinkage cracks in stucco should be anticipated; what is acceptable depends on customer expectations. Following proper curing practices and using good mixes with the right amount of water will reduce cracking. Cracks in the scratch coat get filled by the brown coat. If you see larger cracks in the brown coat, however, it's best to fill them before proceeding with the color coat. Hairline cracks (anything smaller than 1/16 inch) in the color coat do not allow water intrusion, so it really serves no useful purpose to patch them. The patch will inevitably be a different color and will only detract from the beauty of the stucco. Structural cracks -- cracks that go all the way to the back plane of the plaster -- are another story. Although the weather-resistive paper should prevent water from getting to the framing, it's best to patch any crack that goes all the way through the scratch coat. As mentioned earlier, the best cure for these cracks is prevention: Make sure the foundation doesn't settle and use dry, straight framing materials. Ron Webberowns and operates Prime Plastering in Norco, Calif. He has worked as a plasterer for more than 30 years.