This overlay was colored with topical dyes.
Acid stains: Paint and concrete go together like oil and water. And not only does paint bond poorly to concrete surfaces, it also obscures the natural imperfections and variations of the substrate that are a key part of concrete's aesthetic appeal.
by David Thompson
, concrete is still the dull, gray stuff of sidewalks and freeway overpasses. But as a growing number of designers, contractors and customers realize, concrete doesn’t have to be the Ugly Duckling of building materials.
Color can be added to concrete as it’s mixed, using special integral colorants, or it can be added to freshly poured concrete as it’s drying, using powered color hardeners. But some of the most creative effects are achieved after concrete has hardened, and sometimes years after it was poured.
For hardened concrete, there are three basic coloring options: acid stains, dyes and acrylic-based stains. Painters who understand these materials can participate in the expanding field of decorative concrete.
Paint and concrete go together like oil and water. And not only does paint bond poorly to concrete surfaces, it also obscures the natural imperfections and variations of the substrate that are a key part of concrete’s aesthetic appeal.
“That variegation is what everybody wants,” says Chuck Brunner Sr., president of Smith Paints. “Very few people want the painted look on concrete. Most are looking for the natural finishes that look like stone.”
Acid stains are popular and widely used by concrete artisans. They produce translucent colors that not only complement concrete’s natural variations, but also add to it.
An acid stain consists of metallic salts suspended in a solution of acid and water. The acid opens the pores of the concrete, allowing the salts to enter and chemically react with the deposits of free lime they find. Water fuels the reaction. Since the lime is deposited unevenly throughout the concrete, the intensity of the reaction varies across the surface. This results in the variegation, or mottling, that acid stains are famous for. It also adds an element of unpredictability to the results — which makes doing a sample on an unobtrusive area especially important.
Acid stains typically penetrate the concrete from 1⁄16 inch to 1⁄8 inch, and they last forever — or at least as long as the surface of the concrete lasts. The chemical reaction between stain and concrete creates a permanent bond between the two, which makes acid staining a good choice for areas with a lot of traffic or exposure to ultraviolet rays.
While one manufacturer’s line of acid stains will vary in color from the next, all are variations on three basic color groups: blue-green, brown and black. These color choices combined with the variegation effect lend an organic look to acid-stained concrete.
There is a wide range of approaches to applying acid stains. They’re commonly sprayed on using airless or other types of sprayers (just make sure there are no metal pieces coming in contact with the acid), then vigorously scrubbed in. A wide variety of faux techniques, such as ragging and sea sponging, are also commonly used. Layering of different colors, or different dilutions of the same color, can be done to achieve depth and contrast.
The importance of surface preparation can’t be overemphasized for acid stains. Any contaminants — be they grease, oil, taping compounds or whatever — must be removed or they will affect the stain. Common approaches include grinding with a diamond pad, or sanding using a buffing machine fitted with #60, #80 or #100 paper or screen-mesh sandpaper. Consulting the stain’s manufacturer can help.
“Surface prep is extremely important before applying stain,” says Scott Thome, director of product services for Scofield Systems, a manufacturer of decorative concrete supplies. “If there’s any sheen at all, don’t apply it. The sheen has to be knocked off the floor.”
It’s also critical to protect eyes, skin and lungs when using acid stains.
Once an acid stain has finished reacting, it needs to be neutralized or the residue will continue to burn skin. Baking soda, TCP or ammonia is typically used. Once that’s done, the stain’s residue needs to be completely removed so the sealer will bond properly. A scrubbing machine, water mixed with detergent, and a wet vacuum are typically used for cleanup. The residue is a hazardous waste, and should be disposed of properly.