(Elastomeric Coatings vs. Acrylic Latex)
“Stucco” is a mixture based on Portland Cement that is widely used as a siding material on Texas homes, as well as other parts of the country. Stucco may be applied by hand trowel, sprayed on to a very coarse rough texture, or sprayed on and then partially smoothed with a trowel to created a ‘knock-down” or “drag” look. In some parts of the country, a tint is mixed into the stucco mixture so that it does not require painting, while in other parts of the country, stucco is routinely “painted” with an elastomeric coating. Elastomeric coatings are very thick, “rubbery” materials meant to provide a waterproof coating to the stucco.
Wherever freezing conditions are likely (and yes, though it’s hard to believe during the summer, winter temps in Texas often drop below freezing), elastomeric coatings are typically used. The reason for this goes back to the physical property of water expanding when it freezes. If “raw” stucco gets water soaked and the temperatures drop below freezing, the stucco will be damaged as the water expands when it freezes. So by coating the stucco with an elastomeric coating, it is made waterproof. This means that damage caused by water penetration followed by sub-freezing temperatures becomes a non-issue. So the question then becomes when and how to re-coat stucco.
The “when” is the easier of the two questions, so let’s take that first. Of course, if you desire to change the color, you can re-coat any time. In general, the life expectancy with stucco coatings is 5 – 10 years. As elastomeric paints break down due to weather and age they “chalk out”. If you rub your fingertips across the coating and pick up a chalky residue reminiscent of running your fingers across a chalkboard, then the elastomeric is breaking down and no longer doing its job. This means it is time to paint. Substantial or uneven color fading is also an indicator that it is time to paint, as is the existing paint bubbling or peeling. If the stucco is to be re-coated with an elastomeric coating (more on this in a minute), then temperatures should be above 60 degrees due to the longer drying time of these coatings.
As for “how” stucco should be painted, there are two main approaches, and which one is best depends on the existing surface condition and how long the new coating needs to last. Repainting with an elastomeric product is always a viable option, but as mentioned above, the life expectancy is only 5 – 10 years, and most recommend a two-coat application to achieve that life span. As with other paints, there is a range of products available to meet a range of needs. Some premium grade elastomeric coatings available may give closer to 10 years than 5, and with only one coat application. Expect to pay more for these materials, but enjoy a greater savings in reduced labor. If a life span of greater than 5 – 10 years is desired, then a high-quality acrylic latex product will bond to the elastomeric and so may be useful, but there is one important thing to consider – “pin holing”.
Because of the texture of stucco surfaces, paint must be “back-rolled” when applied. Back-rolling is the act of using a roller to squeeze the paint into the voids and crevices in the stucco surface. “Pinholing” is a problem that occurs when applicators who are in a hurry try to rush the job and vigorously move the roller faster than they should. This introduces air bubbles into the freshly applied elastomeric, and when these air bubbles pop, they leave “pinholes” in the coating. These “pinholes” are an entry point for water that can cause premature bubbling or peeling of the elastomeric coating. If these pinholes are present, then simply top-coating with an acrylic latex paint is not advised.
High-quality acrylic latex paints will stick to the existing elastomeric coating just fine, but they will not fill the pinholes. This is because the paint will not fill the holes and as it dries it shrinks and the holes re-appear. The means the freshly painted surface will still have pinholes that are entry points for water that can lead to blistering and peeling of the new paint. The solution to this is to use a “block-filling” primer that solves the issue of pinholes. The primer will bond to the elastomeric and fill the pinholes at the same time. Then, an acrylic latex top coat may be safely applied. Of course, if there are no pinholes present, then the acrylic latex may be applied directly over the existing elastomeric (once it is properly cleaned and prepped, of course).
So to summarize, elastomeric coatings may always be used over top of existing elastomeric paint, but even a high-grade elastomeric will only last 10 years, give or take. If increased longevity is desired, a premium grade acrylic latex paint may be applied, but if the surface exhibits “pinholes” priming with a “block-filling” primer is needed to fill the pinholes. Each of these three possible solutions (elastomeric, acrylic latex, block-filling primer plus acrylic latex) has trade-offs with respect to cost and life expectancy that should be carefully considered when making a decision about how to paint stucco.
Copyright 2010 Jeff Stec