Excerpt from The Stucco Book, The Basics by Herb Nordmeyer
“Let’s get away from the chemistry for a little bit and look at a wall that is freshly plastered. IT tends to be a dark gray color. As time passes, some areas of the dark gray fade to a lighter gray. As more time passes more of the dark gray fades to a lighter gray. That dark gray color indicates that there is plenty of moisture present in the mud for chemical hydration to continue. That lighter gray color indicates that there is not enough moisture present for the chemical hydration to continue. That is a slight oversimplification, but pretty accurate.
If the wall dries out, it is difficult to get it wetted down again to re-create that dark gray stucco color. As a result, it is best to moisten the wall as often as necessary to prevent the dehydration. You are not trying to add water to the stucco, but rather to add as much water to the surface of the stucco as evaporates from the stucco.
The rate of hydration of the cement molecules is temperature-dependent. At lower temperatures, the rate slows and almost stops. If the wall freezes during the first 24 hours after applying the mud, the water in the wall turns to ice and expands. That is why ice floats on top of liquid water. When the water in the stucco freezes, it also expands and puts pressure on the stucco. This causes bonds to break and pieces of stucco to come loose. As a result, the stucco should not be allowed to freeze during that first critical 24 hours. After that first critical 24 hours, a portion of the water in the stucco has been chemically combined with the cement molecules. With that and with a minor loss of water from the stucco due to evaporation, there is some space within the stucco, so if the wall freezes, there is a possibility that no frost damage will occur. With each passing day, the wall can handle more freezing.
The first step in successfully curing stucco is to arrange your schedule so you do not have to apply stucco when freezing weather is anticipated or when strong drying conditions are likely to exist. f freezing weather is anticipated, plan the construction of the protection before you apply any stucco. After the stucco is applied, encloses it with protection and add gentle heat. A little heat over a long period of time is more effective than a lot of heat over a short period of time.
In hot weather, plan your day so you are never plastering in the sun. Come out early and start on the west walls so they will have a long curing period before the sun hits them. Then move to the southern walls. After the sun passes its zenith, move tot the eastern walls, and finally finish on the northern walls.
So, how should a wall be moistened? Use a Hudson-type garden sprayer. Wait until the wall is thumbprint hard, so water will not erode it, and then lightly moisten the entire wall. If you use a garden hose, you may erode some of the stucco. Spray the wall whenever the wall starts appearing just a little bit light.”
More details about curing stucco and a variety of other topics including application techniques, lath materials, the chemistry and history of stucco are available in The Stucco Book, The Basics by Herb Nordmeyer.
