What they know that you don't
BY JOHN SANGER
There must be some sort of secret stucco society. If you listen closely when painting industry professionals gather together, it's easy to spot likely members. When the discussion turns to exterior stucco, they grin knowingly at each other -- or shall we say, crack a smile -- and one of them recites what must be the society's secret code for recognizing fellow members. Without fail, someone will say: "There are two kinds of stucco -- stucco that has cracked and stucco that is going to crack."
Then they drift away from the main group, and in hushed voices, talk about the joys and perils of applying coatings to stucco. Although no one has admitted that such a society exists, several painting contractors and coatings manufacturers -- suspected to be highly placed in the organization -- agreed to share some stucco-coating tips with PWC's readers.
First and foremost, most of them said, you have to understand the nature of stucco. In essence, it's an exterior plaster that's applied in up to three coats. The three-coat procedure is the older, traditional way. Some new stucco systems use only a single coat, however.
Stucco can be applied over a variety of materials, ranging from wood to metal lathe to bricks and blocks. The properties of the material under the stucco can affect the performance of the stucco, which in turn affects the coating you apply. That's why it's important to know as much as possible about the structure you'll be coating. It's not enough to look at a wall and say, "Oh, it's stucco" -- not by a long shot.
The cure: Some basic forms of stucco have been around for centuries. It may look innocuous and easy to paint, but don't let it fool you. "One of the biggest problems you'll face with stucco is alkalinity. New stucco has a hot surface. The alkalinity drops if the stucco is allowed to cure from 15 to 30 days, but general contractors and building owners often don't want to wait that long," said Frank Greer, vice president of Nelson J. Greer Painting in Tucson, AZ, and PWC contributor.
The Portland Cement Institute and the Gypsum Plaster Association both recommend that cementitious products, including stucco, cure for 28 days before painting. These organizations also recommend that the newly applied stucco be fogged (periodically spraying with a fine water mist) and kept damp for 48 to 72 hours.
Here's the reason: "If you follow these guidelines, the water you put on the surface fills the capillaries of the stucco, allowing the water-soluble salts to migrate to the surface," said Bill Lunsford, executive director of the PDCA Arizona Council. "The carbon dioxide in the air converts the calcium hydroxide and calcium sulfate -- both of which can have pH levels approaching 14 -- into calcium carbonate, which is pH neutral."
If you work in humid climates, nature takes care of part of the hydration process. If you're in a dry climate -- and the stucco isn't fogged -- you can end up with some hot surfaces. "Even after 28 days, you can still have surfaces with a pH of 11 or 12, and the only thing that's going to bring it down is time and moisture," said Ron Hutton, sales administration manager for Coronado Paint in Edgewater, FL.
If the 28-day curing period has expired and the pH still is on the high side, politely recommend that the plastering contractor or owner spray down the building two or three times a day for a day or two. (Remember, you just paint it; somebody else builds it.) A garden hose with a nozzle and a fine spray mist is all that's needed. "That helps the stucco finish going through the hydration process," Hutton said.
"It's important for a painter to know that the wall he's going to coat has a neutral pH, or as close to neutral as he can get it," said Bill Lunsford. "He should know when the stucco was applied and what system was used. He also should try to find out if the wall was fogged and cured properly. If he knows these things, he can make much better decisions about how to coat the surface."
Another property of stucco, especially with a one-coat system, is its high expansion coefficient, compared to other types of masonry surfaces. With a one-coat system that is only three-eighths- to one-half-inch thick, heat is absorbed quickly and the material expands quickly, which can lead to cracks. Because stucco expands uniformly in all directions, any protrusion, like an electrical junction box or plumbing fixture, can become a problem if it is sitting tight against the stucco when expansion begins.
Although movement occurs more quickly with one-coat systems, the traditional three-coat systems aren't immune. They just move more slowly.
Ideally, cracking is minimized if the plastering contractor properly places the control joints during construction. The world would be a different place if things were more often ideal, of course; so assume that there will never be a shortage of cracked stucco.
Stretching exercises: Stucco's movement and cracking will wreak havoc with a coating if it won't stretch. "Elastomeric coatings are ideal for stucco," said Dan Johnson, field service advisor for Dryvit Systems Inc., in West Warwick, RI. "High-quality elastomerics will stretch over most small cracks. Nothing is going to stretch far enough to protect against a major structural crack, of course."
Although there are no hard-and-fast rules for what size cracks can be coated over and which ones should be filled first, there are some general guidelines:
7 to 32 mils (hairline to one-sixteenth inch): These are considered hairline cracks, and generally can be covered with normal coating procedures.
32 to 125 mils (one-sixteenth to one-eighth inch): You can still use some elastomeric coatings or an elastomeric sealant on a crack this size, but you'll probably want to use a brush-on type of sealant as well.
125 to 500 mils (one-eighth inch or larger): These are serious cracks, so you want to look for the cause to determine whether or not they will grow anymore. If further growth is unlikely, you may want to consider a combination of elastomeric sealant, nylon mesh embedded in an elastomeric coating, or stucco patching. If the cracks are still growing, time for a structural engineer.
To further complicate applying coatings to stucco surfaces, if you're faced with a structure that doesn't have an overhang or isn't capped properly, you'll find moisture intruding into the wall behind the stucco and working its way into material itself. That can cause any of several problems.
As you know, when water gets into stucco the efflorescence process starts, allowing salts to migrate to the warmer side of the wall. If the alkali-laden moisture can't escape, it will cause hydrostatic pressure to build up under the coating. If hydrostatic pressure isn't a problem, then the high alkalinity of the escaping moisture might affect the coating system's bonding properties. At best, if the moisture and salts manage to escape through the coating, chalky efflorescence will be left on the topcoat.
Don't be misled into thinking that because you operate in what is considered a dry -- or relatively dry -- climate that moisture won't be a problem. Even if rainfall is scarce in your area, you probably have dew that forms during part of the year, generally from October through April. If the dew has any point of entry, through improperly installed, worn, or damaged flashing or the tops of parapets, for example, you can have as many problems as if you were in a wetter climate.
Prime importance: Alkali-resistant primers have their uses, but don't think they're an easy ticket out of letting the stucco cure properly. "Most painting contractors know that the primer is the important part of a paint job," Frank Greer said. "But even if you use an alkali-resistant primer on stucco, what you've done is lock the alkalinity into the substrate, and it's never going to become neutral. These special primers take care of the problem initially, but when a crack develops, watch out, because efflorescence will pour out."
If you understand the characteristics of stucco and the coatings systems designed for it, then you stand a much better chance of having a problem-free job. "Find the existing cracks and determine what caused them. In severe cases of cracking and water entry, you may need a true waterproofing material," Dan Johnson said.
The bottom line on coatings stucco, straight from the secret society: When it comes down to the nuts and bolts of making a decision of whether or not to paint, there are two considerations: moisture content and alkalinity. If you have those under control, the job will go a lot smoother and the coating system will perform as it's supposed to.