Cement-based overlays aren’t just for floors. You can mimic natural stone, block, brick, leaves, tree bark, animal footprints, seashells, and even fossil renderings quickly and economically with resurfacing products designed for wall applications. They’re lightweight blends formulated to be applied up to 3 inches thick without sagging. Because they go on so thickly, you can create deep rock textures and other designs using stamps or hand-sculpting techniques. Accent coloring with stains or dyes completes the effect, making it possible to reproduce the multitoned, weathered look of natural stone or aged brick.
Vertical overlays can be applied to virtually any primed wall surface, including concrete or masonry walls, cement-based backerboard, insulating concrete forms (ICFs), plaster, and drywall. Popular applications include interior accent walls, fireplace fronts, storefronts, entryways, retaining walls, exterior privacy walls, foundation walls, and chimneys.
Preparing the Surface for a Tight Bond
For a vertical mix to adhere properly, you must follow the manufacturer's recommendations for preparing the wall. First, clean the surface to remove contaminants. Pressure washing is an effective way to clean exterior concrete and masonry walls.
If the wall is badly cracked or unstable, reinforce it with expanded-metal lath. I also recommend using metal lath to reinforce exterior walls subject to severe freeze/thaw conditions to ensure a stable support system for the overlay. You can attach the metal lath to the wall with an air-driven nail gun or fasteners.
If the wall is extremely smooth, you may need to lightly profile the surface to improve bonding. Methods include sandblasting or application of a gelled acid, an etching product that clings without dripping.
After washing the surface and allowing it to reach a saturated surface dry condition, apply a liquid acrylic bond coat as a primer. Without this, the wall could absorb moisture from the overlay and cause the mix to set too quickly. After primer application, some manufacturers recommend applying a thin scratch coat of the mix to improve adherence of subsequent coats.
Mixing: It’s All About the Ratio
The proper ratio of liquid and dry components is imperative. You want a workable consistency with enough viscosity to hang onto the wall without sagging. If too dry, the mix could crumble off the wall because not enough moisture is present to help it congeal and cling to the surface. If too soupy, the mix will run down the wall. Different manufacturers may have different mix ratio requirements, so check technical data sheets for proper proportions.
Mixing procedures are similar to those for horizontal overlays that will be stamped. First, add the appropriate amount of liquid polymer to the mixing bucket or pail. If integral color is to be used, add it to the liquid and mix for about 30 seconds. Slowly dump roughly half the powder into the vessel while mixing with an industrial-type paddle drill running at a minimum 600 rpm. After mixing for 30 seconds, add the rest of the material and mix until thoroughly blended, approximately 3 minutes to 5 minutes.
On commercial projects requiring large volumes, follow the same sequence using a mortar mixer.
Some manufacturers recommend letting the mixture set for a few minutes to allow the cement particles to fully absorb the available moisture. During this time, called an induction period, the material will stiffen. After the induction period, it's usually necessary to re-drill the mix while adding more liquid polymer until you obtain the right consistency. This produces a more workable mix that won’t set as quickly.
Applying the Scratch Coat
Some vertical mix manufacturers recommend applying a scratch coat of the material before applying the final texture coat. If this is the case, apply it over the previously primed walls while the primer is still slightly tacky.
Application method depends on installation size. One way is to use a hand trowel or a rubber sponge float (commonly used for applying tile grout) to apply the coat in a light, uniform layer no thicker than 1/8 inch. On large projects, you can cover more area faster by using a hopper gun or an industrial spray unit commonly used to apply shotcrete.
Don't let too much time pass before applying the texture coat. Typically, if more than 2 hours have gone by, you'll need to apply another light layer of scratch coat.
Applying the Texture Coat
Hand troweling is usually the preferred application method for texture coats. However, some professionals use more creative methods, such as a terry cloth rag—a great way to build up thicker layers when creating faux boulders, for example. On large projects, you can also apply the texture coat with a shotcrete-type spray unit.
Don't worry about smoothing it out because you'll be imprinting or sculpting this coat later. It is, however, important to apply the material at uniform thickness to achieve a consistent stamp impression.
Pattern profile dictates application thickness. If the stamp is lightly textured, you may get by with ½ inch. A pattern with a deeper profile and distinct grout lines requires a thicker application.
If the final thickness you want to achieve exceeds 1 inch, I recommend applying the mix in two builds. For example, to obtain an overall thickness of 2 inches, apply a 1-inch layer and let it set for 15 to 30 minutes. Then go back and apply an additional inch. Don’t let the first coat dry before applying the second one; you want the applications to dry together as one coat to prevent separation.
Stamping Tools and Techniques
Although some mats used for horizontal stamped concrete are suitable for vertical stamping, you can obtain more realistic results with less effort by using tools designed specifically for vertical applications. They’re usually made of urethane foam or rubber and smaller and more pliable, enabling you to press material into all crevices of the stamp and achieve deeper and more distinct patterns. Stamps for deep patterns are sometimes made of steel and come with texturing skins made of flexible rubber; pre-texture the wall with the skins and then use the steel stamps to form the indents for the grout lines.
Like those used on horizontal surfaces, vertical stamp sets are commonly numbered to avoid a repetitious pattern. You also must apply release agent to keep stamps from sticking to the freshly applied mix, but use a liquid rather than a powder.
There are many variables in determining proper stamping time. Sometimes you’ll start right after application, some applications require a waiting period. Primary considerations are temperature, wind conditions, coverage area, and thickness. If the mixture starts to sag after you stamp it, you began too soon. If you see tearing at the edge of the stamp pattern, you waited too long.
Bottom to Top or Top to Bottom?
On some jobs, you'll get better results if you start stamping from the top of the wall working your way down, while others may necessitate stamping from the bottom up. It depends on the levelness of the wall and where you want the pattern cutoff to be less noticeable.
On a retaining wall with a cap already in place, you can achieve a full course of brick, block, or stone by starting at the top and working your way down. In this case, the pattern cutoff will be less noticeable at the bottom of the wall. Conversely, if you're stamping a vertical surface that starts from a level slab, starting at the bottom is usually best.
Carving Joints By Hand
The "carveability" of vertical overlays is a big advantage, especially when you want to achieve stone or masonry patterns with deeper reveals and grout lines than possible with stamps alone.
You can use carving tools similar to those used for carving clay. Begin 2 hours to 6 hours after placing the final texture coat. If material adheres to the carving tool, it's too soon and you need to let more time pass. If you get raveling, use a small paint brush dipped in water to smooth out imperfections.
Coloring a Vertical Overlay
As with the other cement-based toppings, many coloring options are available to enhance your vertical stamping job. The quickest and easiest way to introduce color is to add a liquid or powdered pigment during mixing. Other options include dyes, paints, acid- and water-based stains, and tints.
I prefer water-based stains or latex-based concrete paints because of their ease of application. They come in an unlimited palette and are easily blended to achieve custom hues.
Before adding pigments, check with the mix manufacturer to confirm the products you plan to use are compatible with their system. If you plan to stain the wall, allow the freshly applied mix to cure at least one day, longer in some climate conditions. If applied too soon, some stains adversely affect the wall surface. It's also wise to wait a day or so before applying acrylic- or latex-based paints to allow some moisture to evaporate and reduce the chance of moisture becoming trapped within the wall.
You can choose from a number of methods to apply accent color, depending on the effect you’re going for. A simple technique I like to achieve a mottled or antiqued appearance is to apply an accent stain by spray and then immediately wipe the stain off the high areas. You can also use traditional faux finishing techniques such as sponging or ragging, stippling with a brush, or hand-painting individual stones.
Seal the stamped wall to preserve the work and enhance the color.
