Overlay thickness is dictated by stamping mat or texturing skin profile. The rule of thumb is to make the topping twice as thick as the stamp’s deepest indent. Example: Deepest indent is ¼ inch; apply overlay ½ inch thick.
Adobe Stock/titikul_b Overlay thickness is dictated by stamping mat or texturing skin profile. The rule of thumb is to make the topping twice as thick as the stamp’s deepest indent. Example: Deepest indent is ¼ inch; apply overlay ½ inch thick.

Stamped overlays are a great way to rejuvenate interior or exterior concrete, especially where removal and replacement is cost-prohibitive. In addition to providing the aesthetic attributes of stamped concrete, overlays are less time- and labor-intensive. They also enable restoration without messy demolition and risk of damaging surrounding structures, such as a swimming pool coping or stair and wall surfaces; and beautifully mimic other paving materials. Stamping mats and texturing skins are available in dozens of patterns, enabling you to offer customers the beauty and texture of natural stone, brick, slate, cobblestone, and even wood planking without the expense.

The key to success is knowing what not to expect from the plethora of systems on the market. Some are single-component products that only require water; others are two-component systems requiring a dose of liquid polymer. Some involve a third component, like cement or sand. Mixing and installation guidelines differ. Procedures that work well for one system may not work well for another. Don’t assume systems are interchangeable.

Regardless of system, proper substrate preparation optimizes performance. Take into account moisture-related issues, repair cracks, and profile the concrete. Because stamped overlays must be thick enough to imprint, mechanical profiling methods like shotblasting are recommended. If that’s not possible, some manufacturers recommend rotary scrubbing and then acid washing. Be sure to neutralize the acid and remove all residual solution by pressure washing.

Preparing for a Stampable Overlay
As with any resurfacing system, never apply a stampable overlay to a raw concrete surface. Most require a primer of slurry skim coat to act as a bonding agent and keep the concrete substrate from absorbing moisture from the freshly placed overlay, causing it to flash set. Follow technical data sheet recommendations. Many say to let primer dry to the touch or until it’s tack-free, typically after 1 hour to 4 hours depending on ambient and substrate temperatures. Others, however, warn to never let the freshly applied primer dry before overlay installation. Number of primer coats also varies; some systems require one be applied the night before or 8 hours to 12 hours before overlay installation and another right before overlay installation.

Instead of primer, some overlays require prewetting to obtain a saturated surface dry condition. If so, wet with a soaker hose or sprinkler for a minimum of 4 hours to 8 hours to completely saturate the concrete. Don’t rely on a light mist right before installation because that could cause the overlay to set before you can stamp it.

Either way, never leave puddles of standing water or primer. Use a paint roller, rags, or a leaf blower to dry damp areas before putting down the overlay.

Mixing a Stampable Overlay
Although most mixes are prepackaged and premeasured, the mixing process isn’t foolproof. It’s easy to make mistakes that could undermine installation.

For starters, proper volume of liquid (water to polymer) is required to ensure correct consistency. Too much liquid could promote shrinkage cracking and a weak surface; not enough could jeopardize overlay adhesion and workability. You also must mix materials thoroughly without overdoing it, as insufficient mixing could entrap air and cause bubbles to form.

Installation size determines mixing method. On projects of 2,000 square feet or less, I mix materials in 5-gallon buckets or 30-gallon barrels that hold two or three bags and mix components using an industrial-type paddle drill. (Make sure you have a backup drill in case one burns out during mixing).

A mortar mixer that holds four to five bags (roughly 3 cubic feet to 6 cubic feet of material) enables mixing of larger batches for commercial projects. Dump it into wheelbarrows to get the product from the mixer to the work area.

How Thick? Applying the Overlay
Stamped overlays require more skill than other toppings because of their thicker consistency and additional steps required for stamping and finishing. They’re typically ¼ inch to ¾ inch thick, depending on stamping mat or texturing skin profile. The rule of thumb is to measure the stamp’s deepest indent and make the topping twice as thick.

  1. Spread overlay with a gauge rake, a tool similar to a garden rake but with an adjustable depth gauge for distributing toppings to desired thickness. Do this while the material is still viscous enough for spreading. If the mix is too dry or setting too quickly, the material drags along in a clump rather than adhering to the surface. Don’t worry if the rake leaves track marks; you’ll trowel those out later in preparation for the stamping base.
  2. Trowel surface using a steel hand trowel or fresno. Move the blade in a slow, continuous motion to consolidate the fine aggregate while bringing a layer of paste to the surface. One pass is usually sufficient. Overworking can cause “density blisters:” raised domes where air or moisture is entrapped underneath the surface layer. It’s OK if the surface isn’t perfectly smooth because you’ll be imprinting it later with the stamping tools. If the material seems sticky and you have difficulty smoothing it out, some manufacturers suggest misting the surface with water to lubricate the finishing trowel. Use a light layer so you don’t adversely affect surface strength and impression quality.
  3. Apply release agent to stamping mats and overlay surface. Release agents come in dry or liquid form.

The powdered form, which is available tinted, is sometimes called an antiquing release. Usually the stamped overlay is integrally tinted with one color and antiquing release is applied in a contrasting color to produce a slightly mottled, weathered appearance.

Broadcast powdered release agents over the entire overlay surface by hand or by flicking it onto the surface with a dry Tampico brush about 8 inches wide. Be sure to check product literature for coverage rate. Typically, one 30-pound pail covers about 1,000 square feet (application rate of 3 pounds per 100 square feet). The goal is to apply the release in a light, uniform layer. Too much buildup could interfere with imprint texture.

Powdered release agents do a wonderful job of breaking the bond between the stamps and surface mortar but cleanup can be messy, especially if your project is indoors. Generally, 70% to 80% of the agent should be removed after you stamp the overlay and allow it to cure. Removing most of the release helps you obtain subtle color tones and ensures proper sealer adhesion.

Liquid release agents are more common with stamped overlays. They also provide good bond- breaking capabilities and removal is usually unnecessary because most of the product evaporates. However, they’re clear. Add color by mixing ½ cup to 1 cup of tinted powdered release into 5 gallons of liquid release. It’s best to add the powdered release at least one day before overlay installation to allow the powder to dissolve completely. Use a pump-type sprayer to put down a uniform layer right before you stamp. Agitate the sprayer frequently by gently shaking so the pigment doesn’t settle to the bottom and create dark splotches when sprayed on the overlay.

Next time: How to ensure everything goes smoothly once you begin stamping. Read more Bob Harris “Troubleshooting” columns here.