More About Stucco
Can stucco (portland cement plaster) be applied directly over painted brick?
This is a common question that often arises when people are rehabbing or updating older construction. Plaster is a cost-effective finish, relatively easily installed, that improves the appearance and creates a water-resistant wall surface.
A painted surface will not typically absorb water and, as such, is a substrate to which stucco will not readily bond—at least not uniformly. There are two basic alternatives to covering a painted brick surface with a new coating of portland cement plaster.
Sand blast or water blast to remove the paint in its entirety, then direct apply a two-coat system. It is essential to have a surface that is uniformly absorptive to accept the plaster coating. In addition, it may be beneficial to use a bonding agent or dash bond coat with this approach.
Attach paper backed lath or install appropriate building paper between wall and attached metal lath to provide a moisture barrier and to serve as a bond breaker. Apply traditional three coat stucco to metal lath and accessories. In this approach, the idea is to treat the plaster like a sheathed system, using metal lath to support the plaster on the substrate, while completely isolating the plaster layer from the backup with building paper. This prevents a partial bonding situation, which could set up undesirable stresses in the plaster and lead to cracking.
Does stucco require curing, and if it does, how is this best accomplished?
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 inch 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.
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.
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.