ENHANCING CONCRETE with acid staining creates a unique and individual look. Acid stains react differently with every concrete surface and add permanent color to existing concrete substrates.

No other system for coloring concrete floors is as wonderfully unique as acid staining. It never looks the same, and this unpredictability is its beauty. Once you learn to anticipate the variables, you can be extremely creative with designs, patterns, and looks. With acid stains, hardscapes and features can take on an aged patina. When used in conjunction with decorative saw-cutting, grouting, imprinting tools, and embossing skins, acid stains make concrete surfaces look like brick, slate, stone, tile, or terrazzo.
     
Working with acid stains isn't easy. But with the right training and preparation, contractors can make a profit. Beginners should start with small projects in order to avoid making expensive mistakes.

Unlike paint, stains are unpredictable. They react differently with each concrete surface, regardless of age. Acid stains add permanent color to existing uncolored and colored concrete substrates. Because of their translucency, they color the surface without hiding it. This quality enables some of the imperfections of the substrate to show through and often results in a mottled and irregular appearance that closely resembles the shadings of nature.

When staining concrete, you learn to work with what happens, when it happens. Because stain reacts to the free lime in the substrate, you can never be certain how it's going to look. An unplanned drip can become a permanent feature. On the other hand, staining can correct "mistakes" by making them a highlight of the overall design. While it won't correct structural problems, it can visually repair the look of cracks and other surface problems already present in the concrete.

Before laying out a pattern, test the substrate. If the surface is contaminated, clean it thoroughly before applying any stain. The cleaning method depends on how soiled the surface is. Remove all coatings, paints, waxes, water repellents, previously applied adhesives, and curing membranes. Then check for absorbency. In most cases, if the substrate darkens and absorbs the water, you have a substrate that will accept the stain. If the water beads up, some type of coating is present.

Before applying the acid stain, protect the surrounding areas, landscaping, and adjacent surfaces. Rope off the work area; remove nearby vehicles; and close appropriate sections to traffic. Wear suitable protective gear, and follow all government regulations, manufacturer's instructions, and applicable safety requirements.

Coverage and application

Coverage rates on acid stained projects vary from substrate to substrate, sometimes dramatically. Indeed, no load of concrete is the same. You can pour four truckloads the same day and find subtle variances that contribute to differences in the way the stain reacts. The concrete's finish also influences stain coverage -- tightly troweled concrete absorbs stain differently from broom-finished surfaces. The concrete's age and chemical makeup also impact the way it receives the stain. In general, expect coverage of about 150 to 200 sf per coat per gallon of stain.

The best way to put down a reactive stain in a large area is to spray it on with an acid-resistant sprayer, such as a garden-type pump sprayer. This is particularly true for large areas finished with the same color and technique. To obtain unusual finishes, some contractors use faux techniques to apply the stain. Rags, sea sponges, and splattering create unusual effects. Some artisans take the process a step further and use fertilizer, sawdust, metals, cornflakes, and even noodles to add variety to the surface.

These techniques are time-consuming and not cost-effective for large areas. Recommended for experienced contractors accustomed to working with unpredictable techniques, these methods require detailed work on your hands and knees and work best in contained spaces.

No matter what application method you use, keep in mind that you're working with a reactive stain. If the tip of your sprayer clogs and spews out drips, the drips can become a permanent feature of your floor; so always protect the areas around your work surface and keep all your tools clean.

Finishing

Cleaning and neutralizing the stained residue remains the most important step to finishing an acid stained surface. Many contractors clean and seal a surface only to come back and find that the sealer has failed. Using the white cloth test prevents this costly mistake. Simply dampen a clean white cotton cloth with water and run it across the cleaned surface. If the rag soils, the surface is not free of residue. Clean it again, and repeat the white cloth test.

Most floors with saw cuts and stains look better with grouted saw cuts. First, apply at least two coats of sealer and allow them to dry overnight before grouting. When you seal first, then grout, the sealer acts as a resist and prevents the grout from marring the floor design. After the grout sufficiently cures, use burlap, denim, or a buffer to remove the excess. Then finish the floor with another coat to seal the grout as well.

Maintain decorative stained concrete floors with mop-down floor finishes that preserve the sealer. Don't use paste wax. Typical maintenance involves returning with a rotary buffing machine to remove scratches and scuffs, then reapplying floor finish and polish.

Robert Harris is the product training director for a manufacturer of engineered systems for architectural concrete and conducts training seminars around the world. He holds three ACI certifications and is a Board Member o f the ASCC's Decorative Concrete Council.