Probably more vertical overlay cement finishes have been installed than any other type of decorative overlay. It has become a favorite way to finish millions of square feet of commercial building and warehouse walls. For the increasing number of concrete homes, which typically have polystyrene board insulation as the exterior wall surface, thin decorative overlay finishes make an ideal covering. Overlays can be used for both interior and exterior finishes and can be installed directly over concrete, drywall, or polystyrene insulation boards. Most products are rated for both vertical and horizontal surfaces and can be applied by trowel, hopper gun, or sprayer. They can be integrally colored or painted.
Some things you should know
Though some products look like overlay cement, Gary Jones, owner of Colormaker Floors, Vancouver, says that they may not include any portland cement. They are made with polyvinyl acetate (PVA) polymers, fine silica aggregate, and fillers. Generally speaking, these products are more flexible than, but not as hard as, cement-based products so they offer fewer decorative possibilities.
When you are making decisions about which of the many available products to use, consider moisture vapor transmission properties. This is especially important for exterior applications. Jones thinks you should always select a product that is breathable, meaning that moisture vapor from a substrate can find its way through the overlay material and escape into the atmosphere. Products that use acrylic polymers generally meet these criteria.
Thin overlay applications can range from 1/16 to 3/16 inch thick. Terry Viness, senior technical services manager for Sto, Atlanta, says that for non-cementitious coatings, the size of the largest aggregate in a mix usually determines how thick an application should be. Cementitious coatings are generally limited to 3/16 inch to protect against check cracking of the coating as it dries.
Suggestions for installing vertical overlays
In terms of preparation, vertical overlays are very forgiving. Viness says that surfaces must be clean and free of laitance, dirt, and oils in order for the overlay to bond to the substrate. On polystyrene board applications a polymer-modified cement basecoat is normally installed first, along with fiberglass reinforcement to supply rigidity for the finish coating.
Using paper or plastic stencils to add pattern is becoming more popular, especially for simulated brick finishes. Stencils are cut to form simulated mortar joints, and a brick red overlay is sprayed on. Viness suggests precutting rolled paper stencils and laying them flat for awhile before attaching them to a wall. This will help them to rest flat against the wall surface. When you spray overlay cement against the stencils, hold the sprayer perpendicular to the wall so that material doesn't creep underneath the stencil. Start spraying with low pressure and a light coverage of material to flatten and seat the stencils. Follow this with a thicker application. Some contractors apply a vertical broom finish afterwards to further replicate the look of a brick wall.
Timing is important: A stencil should be removed when initial set occurs—before it gets too hard. This will help to avoid edge flaking on the simulated bricks. If sprayed material gets under the stencil at some points, it's best to allow the mix to set and harden overnight and then work the simulated mortar joint with a stiff bristle brush to remove the overspray.
If you use a hopper gun or sprayer, begin by adjusting the sprayer tip size and air pressure on a sacrificial surface, not on a work surface. Move the sprayer in a circular motion to achieve a more even coverage. It's also important to set up a spraying pattern that allows you to constantly spray against a fresh edge of material in order to maintain a consistent finish. This means that you should have mixed material available at all times.
Viness says that generally speaking, overlay finishes are hard after one day, but he advises allowing three days drying time before a final sealer application. Sto recommends that sealers be either water-based acrylics or urethanes.
Thin overlay cement products allow for a wide range of application possibilities so creativity depends on the installer. You can trowel-apply many different effects. Sprayed applications include “knock-down” finishes, “orange-peel” finishes (sprayed overlay cement with spatters of different uniform sizes), and multiple different colored coatings to achieve multicolored appearances. The use of stencils allows limitless creativity.