For cement-based materials, curing is defined as maintaining an appropriate temperature and moisture content for a specific period of time during the early life of the material. All portland cement-based materials, such as stucco, require curing.
Since the addition of water to portland cement sets off a chemical reaction called hydration, it’s important to provide excess water to the cement particles so that they develop a good bond with their surrounding environment: aggregate and other cement particles. This is how plaster (and concrete, mortar, and grout) hardens. Plaster sections are quite thin, ranging from about 3/8- to 7/8-inch total, and individual coats may be only 1/8 nch thick. Thin layers such as this must be protected from conditions that interfere with cement hydration: things that dry them out or heat or cool them excessively.
Wet and Dry
The scratch coat, which is the first base coat, is named for horizontal scratches that promote water retention for curing and a mechanical keying with the brown, or second, base coat.
Sun and wind, alone or in combination, drive moisture out of fresh plaster. To be applied to a wall, plaster must be fluid enough to be troweled, screeded (leveled with a straight edge using a back and forth motion while moving across the surface), and floated, but not too wet that it sags or won’t stick. Base coats, of which there may be one or two (sometimes scratch and brown are combined), can be wetted once they have developed adequate strength so that they are not washed away by the water. Since the coats are thin, they can’t hold as much moisture as is ideal for curing—especially if they are competing with sun or wind, which both cause evaporation.
Plaster can be wetted periodically throughout the day to supply additional curing moisture; usually one or two times per day should suffice. In extreme conditions, sun and wind breaks can be used to provide extra protection from the elements. The first two days are the most critical period. The entire first week is important, however, so it is a common recommendation that the base coat stucco be misted or fogged periodically for the first three to seven days after placement. A sheet of polyethylene can be placed over the moistened surface to hold the water in. If the relative humidity of the air is greater than 70 percent, moist curing may be accomplished without additional wetting of the surface.
A caution about moist curing is that colored finishes can be affected by water application. Finish coat stucco is not moist cured since this may promote mottling and discoloration. Curing of colored finishes is typically done by wetting the base coat to provide curing moisture from behind the finish and ensuring that the surface is shielded from drying.
Colored finishes are usually cured by wetting the brown coat to provide curing moisture from behind. In more extreme conditions, they may be covered to prevent drying.
Hot and Cold
Proper curing also requires that plaster be in a medium temperature range. Usual recommendations range from 40° Fahrenheit on the low side to 90° Fahrenheit on the high side. Too cold and there is a risk that water in fresh plaster would freeze. As this is an expansive process, cracking could occur. Cement hydration can be interrupted, too. Too hot and there is a risk of drying—which, like freezing, can also suspend cement hydration—or of accelerating the hydration process to a point where strength development in the longer term is negatively impacted.
Curing compounds are effective for concrete but are not used regularly on plaster. These materials might interfere with subsequent coats of plaster and might lead to discoloration of the stucco finish.
For more information: Portland Cement Plaster/Stucco Manual, EB049.