A painted stucco finish can create a durable, attractive exterior and greatly impact customer satisfaction.
Stucco is an exterior material used in many markets. It consists of an aggregate, a binder, and water. It's applied wet, and it cures as it dries. As a textured, cement-based exterior, it presents an attractive but challenging surface to a painting crew. The final appearance and long-term durability of a stucco exterior is important to a builder, because it can greatly impact customer satisfaction and the home’s durability.
Science of PaintPaint or primer is a suspension of polymer particles in water. After paint or primer is applied to a surface, the water gradually evaporates, leaving the polymer solids to coalesce (join physically) and bond (join chemically) with each other. After coalescing and bonding, the solids lie flat to create a uniform film on the surface. This process is influenced by how well the painter prepares the surface and applies the primer or paint. For instance, if paint is applied to a dusty surface, the dust particles will interfere with the coalescing, resulting in a lower quality finish.
Surface preparation
The key to preparing stucco for paint is to sufficiently hydrate it during the first few days of curing. Fresh stucco is highly alkaline, and its pH level ideally lowers to 10 as it cures. If stucco isn't properly hydrated, its pH level may be too high for painting. At pH levels higher than 10, the stucco may react to rainfall and splashing with a short-term rise in pH, attacking the paint and breaking it down to reduce its sheen and color uniformity.
After applying the base coat of stucco to the home, complete three hydration cycles on each wall face of the home's exterior. A hydration cycle is a three-step process. To complete one hydration cycle on one wall face, apply water with a flooding spray of a hose until it runs off the stucco in sheets, working from one end of the wall face to the other. After reaching the end of the wall face, return to the beginning, and apply water again until it runs off in sheets. Then, saturate the wall face a third time in the same manner, until the water runs off in sheets. The second and third passes on the wall will require significantly less water to reach the sheeting stage. As the wall hydration progresses, the water will be rejected sooner, requiring less water to be applied for each cycle. Make sure to apply water under porches, at overhangs, and under popouts; normal runoff won’t reach these areas.
Perform the hydration process on all wall faces to complete one hydration cycle. Begin another hydration cycle about two days later or when the stucco has begun to dry. At the end of three hydration cycles, the stucco’s pH level should be lowering to 10. If the pH level stops dropping at 11 or 12, you'll have to use a pH tolerant paint.
During the first hydration cycle, check for areas near trim where clumps of stucco may have formed and remove them. Under areas of exterior detailing, such as detailing above doors and windows, make sure the stucco coating is crisp and clean.
Make sure the ambient temperature isn't too low for painting. Cold temperatures interfere with paint's ability to coalesce and bond; the polymer particles don't have sufficient energy to move together. The air, the surface, and the paint should all be at the manufacturer's recommended temperature, which is usually 40°F and rising. Also, confirm that the temperature won't drop below recommended levels after painting for the timeframe specified by the manufacturer.
ProTip!The humidity of the surrounding air affects stucco hydration. For instance, if the ambient humidity level is 70%, you'll need less water to hydrate stucco than you would at lower humidity levels.
Primer
A good practice is to prime before painting. Primer functions as an intermediary between the stucco and the topcoat. A good primer seals the stucco to provide a more uniform surface for painting. Primer also helps prevent brush and roller marks in the topcoat. The topcoat is applied to the primer, not to the surface. Think of primer as a foundation for a smooth, attractive exterior paint job.
The best practice is to apply the manufacturer's recommended primer thickness, which is usually 2.0 mil dry. Apply a minimum nominal 6.0 mil wet primer coat, backroll the primer, and measure the mil thickness in order to ensure a 2.0 mil dry finish. Apply the paint with an airless sprayer, using the manufacturer's recommended tip size and pressure. Then, backroll the paint with a saturated roller, using the recommended roller type and nap size. Backrolling uniformly coats the stucco texture, spreading the primer evenly.
ProTip!Measure the wet film thickness of each coat by inserting a wet film gauge into the sprayed paint at a flat area, such as at the taped area around a window.
Topcoat
Read the manufacturer’s information on the paint’s solids content, so you can calculate how much paint you’ll need to apply. For instance, if a paint product is 40% solids, you’ll have to apply a 5.0 mil wet coat to obtain a 2.0 mil dry coat. Make sure you don't reduce (thin) the paint.
Stir the paint before applying the topcoat. The best practice is to apply the manufacturer's recommended topcoat thickness, which is usually 2.0 mil dry. Apply a minimum nominal 6.0 mil wet color coat, backroll the paint, and measure the mil thickness in order to ensure a 2.0 mil dry finish. For additional durability, apply a minimum nominal second coat of 6.0 mil wet color coat. Again, backroll the paint, and measure the mil thickness in order to ensure a 2.0 mil dry finish. Apply the paint with an airless sprayer, using the manufacturer's recommended size tip and pressure. Backroll with a saturated roller, using the recommended roller type and nap size. Backrolling uniformly coats the stucco texture, spreading the paint evenly.
ProTip!When painting, work up to—not away from—the wet edge to avoid pulling wet paint away from the surface and to prevent lapping.
Cleanup
Clean oversprayed paint from railings, windows, driveways, and other surfaces immediately after the paint job is done with a damp cloth and soap. Pressure-wash more stubborn stains with warm, soapy water. Cleaning oversprayed areas right away saves time and labor.
ProTip!Before the homeowner orientation, prepare a touchup kit for the buyers to repair scuffs, dings, and blemishes. Include a small quantity of each paint, as well as instructions for use and safety.
