TIPS FOR COATING EXTERIOR STUCCO
The term ‘stucco’ is used to describe Portland cement exterior plasters in a wide variety of textures. While the cement creates a strong finish, the challenge is that it can also be susceptible to hairline or larger cracks—and the resulting problems from water intrusion.
As with all paint jobs, successful coating of stucco is a matter of prep work as well as material choice and application. Larry Baker, owner of Think Stucco in Minneapolis/St. Paul, emphasizes that painters need to think of stucco as a system, not just a surface to be coated. “One of the big problems with painting stucco is that it’s rather airtight,” he says. “When vapor barriers were installed on the inside of walls in the 1970s, half of the breathability was taken away. You want moisture to be able to leave a wall cavity, otherwise you end up with mold, mildew and rot.”
Surface prep starts with a structural examination. Cracks larger than the thickness of a credit card, pits or other minor damage can be repaired with a sealant that’s compatible with masonry and concrete. Blending a larger patch is an art, according to Mark Fowler, executive director of the Stucco Manufacturers Association. “To fill larger holes with a fast-setting cement, you need to start by shaving the surface to below the existing texture,” he says. “The sand size is critical for a good texture match, and 20/30 fine or 16/20 medium are the most common. For a cement finish, wet the base patch and then stipple it with a stiff brush to match.” He cautions against patching by using drywall compounds, spackle or non-cement-based products that aren’t specified for masonry surfaces.
After completing any needed repairs, the cleaning process partly depends on the age of the stucco surface. “Old stucco is going to be more fragile, so you need to use additional care,” says Brendan Steidle, brand manager of primers and specialty coatings at Rust-Oleum. “You should use a compatible commercial cleanser, applied with a push broom, or a light scrub brush to get the crevices clean. If there’s evidence of mold and mildew, you want to apply a biocide with a hose-mount scrub brush, or sponge. If you’re dealing with new stucco, you can use a pressure washer and an appropriate cleanser.”
If you run into a chalky substrate, especially where old paint has flaked off or blistered, be forewarned. “A few decades ago, it was a common practice to apply a white-wash, which is basically just Portland cement with water and maybe some pigment,” Baker says. “Once you go that route, the only solution is to continue whitewashing, because this chalky substrate is not going to be a sound surface for paint to stick to. If that’s what you find, the only answer is to sandblast it.”
PATIENCE BEFORE PAINT
When dealing with old stucco, you absolutely must wait for the surface to be fully dried—at least overnight—before applying primer or paint. New stucco is trickier, and requires additional patience because it’s a matter of chemistry as well as moisture. It may feel dry to the touch long before it’s fully cured, which may take a week or even a month, depending on climate. “When you first mix plaster, it’s very alkaline, with a high pH,” says Baker. “If the can of primer or paint says to wait 28 days before going over plaster or cement and you paint sooner, you’re going to have a bonding problem, maybe quickly or maybe in a few years.”
The exception to the 28-day rule is if you choose to use a so-called ‘hot’ primer. “One option is a block filler, which addresses the high-alkaline surface and allows you to apply prior to full cure, but it’s a thick coating and takes away from the stucco effect,” says Steidle. “If you want to retain the stucco texture, you’re better off going with a basic, masonry-compatible primer.” The primer also serves to bind up residual pigments that can’t be removed by washing.
CHOOSING A COATING
For breathability, experts recommend choosing an alkaline-resistant acrylic or vapor-permeable elastomeric coating for stucco. (Consult the manufacturer for specific limitations or recommendations on surface conditioners or primers.) Acrylics are a little less expensive, while elastomeric may be a better bet for surfaces with a lot of hairline cracks.
“I like elastomeric the best because in my climate, everything is shrinking and expanding, heaving and settling,” says Baker. “There’s not a stucco structure around that doesn’t have hairline cracks.”
Fowler cautions against thinking that elastomeric coatings are a cure-all, however. “You should never use elastomerics to solve water-intrusion issues,” he says. “It tends to only postpone a problem, and it’s always better to fix the problem—which usually comes down to flashing issues. If water gets behind the coating, it can bubble and blister as it tries to escape out as a vapor.”
Flat to semigloss paints are generally the best for stucco in Fowler’s opinion. “Fast-food places like gloss enamel because it’s easy to clean,” he says, “but that’s not really recommended anywhere else because it kills the texture.”
An airless sprayer is ideal for stucco surfaces because it helps get primer and paint into nooks, crannies and crevices. “An airless gets the job done quickly,” says Steidle. “The PSI forces the coating into the crevices most efficiently. You can go toward the higher end of the range of what’s on the label, but don’t go higher than that.”
If you’re rolling, a thick nap of ó” or thicker can help get the coating into the crevices. When using a roller, it may be tempting to think that you need to apply a lot of pressure to the roller in order to achieve a full coat, but Steidle recommends using a thicker nap to accomplish that.
“Most of the concerns we see are in application,” says Steidle. “The store personnel will be able to advise you on the best products and application methods in a given circumstance.”