Stucco is one of the most porous building materials utilized within the construction industry…. an untreated plaster wall will absorb water like a sponge! Many Architects specify colored stucco (color coat) in an effort to achieve a one-step aesthetic effect, but neglect the application of waterproofing. As a result, the continued wetting & drying out process ‘leaches’ the color from the colored stucco much like the repeated washing of a pair of blue jeans. Additionally, certain Architects feel that the application of outdoor paint will act to seal the substrate – their argument is simple, “water will not penetrate paint”.
Let us first address the paint issue …. when stucco cures, or dries after its original application, it has a tendency to shrink and exhibit hairline cracks. Also, as time passes, the building will shift, causing further cracks. True, when first applied, paint will provide a waterproof barrier; however, one must understand that paint does not stretch, and cracks within the stucco, regardless of size, will telegraph outward through the paint. As the painted wall surface is exposed to water or moisture over time, this moisture will enter the stucco through the cracks in the paint …. as time passes, the accumulated moisture in the stucco, behind the paint, will break the paint’s adhesion and the once uniform surface will begin to blister and peel.
Let us not underestimate the damage that water can cause within the stucco wall itself. In cases where water has migrated behind the paint on untreated stucco, water intrusion within the substrate is a certainty . The only failsafe at this point in time is the waterproof building paper back-up which was applied prior to the stucco. In most cases, this paper is either torn or improperly overlapped at time of application, and moisture will ultimately enter the building’s interior over time resulting in interior wall stains, mildew and failure, not to mention the ultimate deterioration of the cement plaster itself.
In all cases, waterproofing products should be applied to cement plaster! In cases in which paint is specified for use, the application of a water repellent product must take place prior to painting. When addressing the issue of product selection, Combocrete KL-23 or Combocrete KL-25 should be recommended. A viable alterative to the application of clear liquid waterproofing and paint upon stucco is the utilization of an elastomeric coating. Widely specified by Architects, Combocrete KL-18 provides a ‘stretchable’ protective coating which resembles paint in its color and appearance.