How to stain concrete
Because it’s porous and neutral in tone, acid-based chemical stains can add rich, earth-toned color resembling wood, marble, natural stone and even leather.
Concrete makes a perfect blank canvas for color. Because it’s porous and neutral in tone, acid-based chemical stains can add rich, earth-toned color resembling wood, marble, natural stone and even leather. Knowing how to stain concrete can be an excellent revenue source.
Most acid stains are a mixture of water, hydrochloric acid and acid-soluble metallic salts. They penetrate the surface of concrete and react chemically with the hydrated lime in the concrete. The acid lightly etches the surface of the concrete, allowing the metallic salts to penetrate more easily. Once the reaction is complete, the color becomes a permanent part of the concrete and won’t fade, chip or peel away.
Each concrete slab will accept the stain in varying degrees depending on the color and condition it’s in at the time of stain application. Traditionally, most concrete stains have been limited to subtle earth tones and soft blue-greens. Newer water-based penetrating stains and concrete dyes have expanded the color palette to just about any color imaginable.
Surface preparation
Whether you’re working on new or old concrete, the concrete slab must be thoroughly cleaned before staining. For new slabs, sprinkle water in several places to make sure the slab absorbs it. If it doesn’t, curing agents may be blocking entry and the stain won’t penetrate.
Older slabs with grease or oil stains must be chemically cleaned and any surface contaminants, such as paint drops, caulk, etc. must be removed. Typically, that can be accomplished by scraping, applying solvents and/or stripping agents, sanding and/or grinding, depending on how dirty the slab is and what types of stains are present.
If grinding is necessary, use a diamond pad or cup grinding head in either diamond or black abrasive. Grind lightly and move in small diameter circles to avoid leaving grinding marks.
The final cleaning should be done carefully with a water and detergent mixture to remove any cleaning agents and acids that could interfere with stain penetration. Scrub with a buffing machine using strip pads and pick up the effluent with a wet vacuum.
Many contractors use grinders or hand-held saws with tables that ride against guides to cut pattern lines in concrete.
Applying the concrete stain
Manufacturers differ on when and how to stain concrete; some say new slabs should cure for at least 28 days before application while others suggest 14 days. Many contractors prefer to begin application as soon as possible after the concrete is placed.
Generally, colors are more intense if stain is applied soon after the concrete is placed, and diluted stains applied immediately often achieve the same results as full-strength stains applied later. Also remember that water drives the chemical-stain reaction. For best consistency, make sure the moisture content of the concrete is roughly the same for every area of placement.
How to stain concrete in patterns
Many designs incorporate decorative cuts and sandblasted patterns to enhance the appearance. Many contractors use grinders or hand-held saws with tables that ride against guides to cut pattern lines in concrete. Use dry-cutting diamond blades that will minimize damage at the edges of cuts.
When cutting before staining, cut just before final surface cleaning so you can remove all concrete dust, which contains lime and can distort the color. When cutting after staining, do it after the first coat of sealer has been applied.
If there is a color change at a pattern line, cut the line first to form a barrier that will keep the second stain color from migrating into the first. If sawed joints will be grouted, complete the staining and sealing before grouting to prevent grout accumulation on unprotected stain.
Sealing
There are several types of surface and penetrating sealers you can use, depending upon your application. Exterior applications should use acrylic sealers that allow moisture in the slab to escape. Solvent-based acrylics generally perform better than water-based for outdoor use. Silicone-based penetrating sealers are recommended if a shiny or “wet” look is not desired.
Choices for interior slabs primarily consist of acrylics, urethanes, and epoxies. Acrylics are UV stable, inexpensive and easy to apply and reapply. However, they have the softest surface and require the most maintenance. Solvent-based acrylics are softer than water-based and provide a “wet” look that enhances many colors.
Epoxy sealers are much harder than acrylics. Water-based epoxies bond well to concrete and provide a clear finish, but they don’t allow moisture to escape. They are not UV-resistant and reapplication is more difficult than acrylics. Epoxies typically are well suited for countertops and food prep areas.
Urethanes are the most expensive and provide the most abrasive-resistant finish. However, they don’t bond well to concrete so they must be applied over water-based epoxies. They are not UV-stable and reapplication is difficult and expensive.
Maintaining the color
Once the job is done, maintenance is usually simple. Clean stained concrete the same way you clean unstained concrete. To keep stained exterior concrete surfaces protected, apply a new coat of sealer every year or two or when water drops no longer bead up on the surface.
—By Rob Fanjoy
