Over the years, stucco has become a top exterior finish choice for homebuilders. It’s well known for its durability, versatility, and cost-effectiveness. However, along with its popularity have come certain issues caused typically by not understanding the recommended industry standards or by cutting corners during the production schedule in order to save time. Cutting corners is risky, and in many cases, it can also be costly. So, it’s imperative to arm yourself and your construction teams with the right knowledge and quality assurance procedures to ensure that every stucco finish is installed correctly.
Here, we’re going to take a look at one area in more detail—the stucco hydration process. Not following properly through with the hydration process can lead to major problems down the road like paint burning, finish coat delamination, and certain types of cracking.
What is stucco hydration?
Hydration is an important chemical reaction that allows stucco to cure properly. During hydration, Portland cement molecules form bonds with each other and with other materials in the stucco mix. As these bonds form, the stucco gains strength. However, in order for these bonds to form, the proper amount of water must be present. Without sufficient water, these bonds won’t form, and the stucco finish will fail.
This failure can show up in different forms—as paint burning, finish coat delamination, or cracking. Inadequately hydrated stucco has an inherently high pH, which will break down paint and reduce its color uniformity. It can also result in a poor bond between the finish coat and previous coat, causing delamination to occur, as seen in the photo to the left. Furthermore, inadequately hydrated stucco can result in cracking that decreases the durability of the stucco finish.
Initially, the water in the stucco mix is enough to begin the bonding process. However, this water is quickly absorbed or it evaporates. In order to keep stucco from losing water, each coat should be hydrated after it’s applied to the wall surface. The one exception to this rule is an acrylic or acrylic-modified finish coat—this type of finish coat doesn’t need to be hydrated.
How do you hydrate stucco?
Hydrate each coat of stucco in the same way. Begin hydration as soon as water can be applied to a coat without eroding it. To hydrate a coat, flood the wall surface with a gentle stream of water, moving from the bottom of the wall to the top until the water runs off in sheets. Work from one end of the wall face to the other. Make sure to apply water under porches, at overhangs, and under popouts; normal runoff won’t reach these areas.
Once the water has been absorbed by the surface of the coat, it’s ready for another cycle of hydration. Hydrate each exterior wall until the water begins to run off in sheets. This process should be conducted up to three times on each wall surface. The second and third passes on the wall will require significantly less water to reach the sheeting stage, because with each pass, the stucco will accept less water. The stucco should retain moisture for the first 48 to 72 hours after it's applied in order to prevent carbonation and to cure to full strength.
What weather conditions affect hydration?
Certain weather conditions can interrupt the hydration process, preventing stucco from curing properly. The ideal temperature range is from 40°F to 90°F. Colder temperatures can cause the water in the stucco to freeze. Hotter temperatures can dry the stucco too fast, reducing its strength. On days where the relative humidity is greater than 70%, the frequency of moistening the stucco can be reduced. In dry conditions, the stucco is in the greatest need of moisture for the hydration process.
Hydration is an essential step in creating a durable stucco finish. When curing occurs properly, it helps improve the wall’s strength and reduce the potential for paint burning, finish coat delamination, and certain types of cracking that decrease durability.
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