Vertical overlay concrete or grout is used to complete vertical rock-work, thinset coatings on walls, stamped masonry unit patterning, free-standing art sculpting, and architectural detailing. They can be built up as much as 3 inches in thickness without sagging or slumping.
Overlay materials are prepackaged products composed of portland or engineered cements, coarse and fine aggregate, and polymer additives that increase the bond to the substrate material and facilitate curing. Some manufacturers add other ingredients as well. Systems can include either dry powder or liquid polymers added during the mixing process.
Liquid polymer bonding agents sometimes are applied first, directly to a substrate to facilitate the bond. Products can be applied and covered with the primary application of overlay mix before they dry (if they dry before being covered they can become bond breakers). There also are polymer bonding agents that re-emulsify when they are rewetted by coming in contact with fresh concrete, so they can either be wet or dry. It's important to know which type of product you are working with. Tom Lundgren, business development manager, Western division for Mapei, San Bernardino, Calif., adds that the challenge for manufacturers is to add the right amount of polymer to their products. “You want to use the maximum amount to take advantage of their properties but too much causes mixes to be gummy and hard to work with,” he says. “Polymer modifiers allow you to reduce the water demand, diminish volume change, and retain water after initial set for curing purposes.” He notes that most polymer modifiers are acrylic based—allowing water vapor to move through but not liquid water.
Building the desirable thickness
Substrates must be prepared so they are porous enough for overlay compounds to bond, especially thicknesses more than 1/8 inch. Then, in accordance with manufacturer instructions, a polymer bonding agent is applied and either allowed to dry or covered with a scratch coat before drying. Scratch coats are heavily textured with tine brushes or other ways to ensure a good mechanical bond for the next application. Applications are usually applied by trowel. Depending on the desired thickness, mixes can include large aggregate. “Mixes for thicker applications usually include lightweight aggregate to keep the weight down,” says Doug Bannister, owner of The Stamp Store, Oklahoma City. Lundgren adds that mixes for stamped impressions of brick or rock often contain less polymer additive and more sand so the consistency is less sticky, helping to reduce the amount that clings to the stamps when they are removed.
“Microtoppings are very thin coatings; from little more than paint thickness or as much as 1/8 inch,” says Steffen Plistermann, owner of Artisan Finishes and distributor of Milestone in Seattle. They typically are applied in two or more applications, depending on the final desired finish.
Selecting fine aggregates for microtoppings is important and there are many choices. The thickness of the buildup and whether the application is a primary or a finish coat influences the choice. Aggregate choices include different sizes of quartz and recycled glass, according to Plistermann. “The smoothness of the final finish influences your choice too,” he says.
Substrate preparation isn't as critical for microtopping work as it is for thicker applications. No preparation is needed on primed drywall, brick, cast-in-place concrete, or even over a plastic surface countertop. However, they are sensitive to substrate movement, so work over wood flooring or cement board with seams requires a layer of fiberglass mesh or metal diamond lath, resulting in thicker applications.
When installing microtoppings, follow these steps.
- Consider substrate requirements. Choices range from applying a bonding agent and proceeding with a primary coat to installing and filling mesh to restrict movement.
- Install a primary coat. Depending on the thickness or buildup required, a proper selection of aggregate for the mix becomes important. This usually is a wet application over a previous dry application.
- Install a second primary coat, if necessary. Again, install wet material over dry. Apply the base coat of the finish. Depending on the desired appearance, fine aggregate can be included. Install wet material over dry.
- Final finish. If desired, a final application can be applied paint-thin or with fine aggregate in the mix.
- Seal the finished work. Use a urethane- or acrylic-based sealer to seal the project.
Plistermann says there are advantages to doing wet-on-wet applications, such as the incorporation of larger size aggregates, but they require more experienced installers and he recommends getting training before attempting it. Layers can be integrally colored.
Ake Grunditz, owner of Fine Design, Alameda, Calif., specializes in architectural detail work and builds up overlay thicknesses to as much as 3 inches. He handcarves architectural patterns with very fine 3-D details. To do this, he alters a prepackaged mix by sifting out coarse aggregate to avoid hitting stones in the carving process. He also adds additional polymer bonding agent by mixing it 50:50 with his mix water. “It becomes quite a bit more gooey and sticky but it's ideal for my work,” he says.
Because he needs a porous substrate, Grunditz starts with an application of a wet slurry brown coat rich in polymer over a substrate material)—usually stucco on the outside of buildings or plaster inside. He either brushes or trowels it onto the surface. After initial set but before the mix loses its water sheen, he applies a second coat, which is the one he carves. The goal is to achieve a wet-bond between the two applications. “The second application must be a fairly dry mix so it will stay in place without sagging when applied,” he adds.
All Grunditz's work is original. He creates his own patterns for each job, drawing them out on thick paper and then cutting out tiny slits on pattern lines. Next, he places the stencil on the sticky overlay material and sprays black paint on the stencil. Every slit opening allows the paint to mark the pattern on the cement. Grunditz uses clay sculpting tools along with cutting tools that he created himself to carve his pattern into the grout, removing material to the proper depth. Afterwards, the slow process of shaping and contouring begins. “I typically remove about two thirds of the grout I apply,” he says. When weather conditions aren't too harsh or when the sun doesn't directly shine on his work, product installed in the morning can be worked on all day.
He sometimes returns a day or so afterward to add antiquing color. Applying sealer is the final step.
As one of the overlay categories, the potential application for vertical overlays is just beginning to be realized. In the hands of a creative person, the possibilities are endless.