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Curing Is Critical

If the stucco is mixed properly and applied over a stable substrate with properly nailed lath, then the conditions under which it cures will determine the quality of the finished product. If you want stucco that will last 100 years, pray for a period of cool, damp days — temperatures in the 50s and 60s, no wind, and high humidity. Applying stucco in extremely hot or cold conditions presents a real challenge: Stucco applied in the cold needs to be heated up, and in extremely hot conditions, it needs to be cooled down. Or said another way, if the weather is hot and dry, you’ll want to slow the curing, and if it is cold and damp, you’ll want to speed it up.

Extreme Stucco

When clients call and say "the finish coat of stucco is falling off my house," I immediately suspect a dead-of-winter job. Delaminating (unbonded) finished coats, which fall off the wall in random-sized pieces like peanut brittle, should be inspected for a telltale crystalline pattern on the back side of the finish layer. That pattern suggests a finish coat that was applied over a frozen (glazed) brown coat during winter conditions. What typically happens is that sleet or blowing rain or snow soaks the brown-coated walls. Unknowingly, the stucco contractor applies the finish coat over a difficult-to-see glaze of ice on the brown coat. Regardless of the ambient temperature, if a finish coat is applied to the glazed walls, the bond between the brown and finish coats will be lost.

Cold-Weather Precautions

The bottom line in working in the cold is don’t use frozen material, don’t apply stucco to frozen or frosted surfaces, and don’t mix materials or apply stucco when the ambient temperature is less than 35°F. If you can swing it, gutters and "shoot-outs" to direct the melting snow and rain away from the walls make winter work much easier. Scaffold planks should be pulled away from the walls and tipped up so as not to serve as splash points for rain to pelt the walls in horizontal bands and then freeze in the night. Cement has an inherent degree of salinity, or natural salt content. Adding an antifreeze, such as a calcium-based accelerant, to the mixer in cold weather will delay freezing, but it also heightens the salt content by magnitudes. This greatly increases the likelihood of efflorescence — the migration of salts to the surface — which leaves a powdery white residue. If cold weather is an issue, you can heat the water, warm the sand with kerosene heaters, keep the raw materials covered at night, and work in the sun, however meager it may be. You can also cover or tent a house, but doing so is both time consuming and expensive (Figure 5).

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Figure 5. Covering stucco in cold or inclement weather extends the curing period and protects against a washout in case of rain.

Nevertheless, with large enough heaters, your working season can be year-round, even in the northern states.

Hot-Weather Solutions

When it’s 75°F or higher in the sun and a hot wind is blowing, it should be obvious that any mud applied to an exposed surface will dry far more quickly than normal. And just like concrete, stucco that dries too fast, or flash-dries, is weaker than stucco that is allowed to cure slowly. In extremely hot, dry, or windy weather, all your attention should be aimed at slowing down the stucco’s drying and curing time and protecting it from uneven and excessive evaporation. First, keep the materials out of the direct sun. Sand holds a lot of heat, as does water. If both are cold, the stucco will dry slower. But don’t moisten the sand in an attempt to cool it: Doing so will just confuse your mixing ratios. Another simple way to avoid problems with the sun is to start work early in the morning and plan to apply the stucco so that the crew "chases the shadows" for the rest of the day (Figure 6).

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Figure 6. Working in the shadows is a good way to prevent rapid drying on a hot day.

Working in shadows will slow down drying times. And be aware that scaffolding planks may cast horizontal shadow lines on the wall, causing those areas to dry more slowly. The stronger stucco in these slow-cure areas will have a different rate of absorption than the areas above and below the plank’s shadow. Later, when the color coat is applied, it will be "sucked" to the dissimilar areas unevenly, resulting in a blotchy finish.

Misting and Draping

Your next defense against flash-drying is to keep the surface of the new mud damp. This can be done on small areas and low walls with a garden sprayer. On higher walls and over larger areas, use a garden hose with a misting or fogging head (Figure 7).

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Figure 7. Spraying stucco in hot, dry, or windy weather prevents excessive evaporation.

For the best results, the walls should be "hit" every hour or so until the sun and wind are no longer a factor. Draping new work with Visqueen or burlap also retards the evaporation of water and prevents flash drying. And a soaker hose can be run at the top of the wall to keep the burlap wet.

Flash Curing

Another solution is to use a retardant like Sikafilm (Sika Corp., 201 Polito Ave., Lyndhurst, NJ 07071; 800/933-7452), an emulsified alcohol product that retards flash drying. It is sprayed on the scratch or brown coats soon after application, then washed off before the next coat is applied. Because the product has an orange color, it is not recommended for finish coats. Depending on how fast the scratch coat is curing, it’s possible to use the double-back method: The scratch and brown coats are applied and cured as one system. The brown coat may be applied as soon as the scratch coat is rigid enough to receive the second coat without damage. This is a judgment call. You run the risk of "stacking" too much mud too quickly, thereby increasing the likelihood of checking in the brown coat. If you double-back, misting becomes even more important — I recommend doing it for 48 hours. When it comes to the finish coat, you really have no choice but to wait for a moderate day. Misting a finish coat creates the potential for splotchy walls.