Natural concrete isn't very attractive, and it becomes even less so as it ages and the cold gray patina gets discolored by leaf stains and mold. If this describes your patio, you don't have to settle for it. Staining concrete is not a difficult job, and you have a wide range of colors from which to choose. You can opt for a sealer to lock in the stain and give the surface a sheen, but you don't really need one; with or without a sealer, concrete stain is permanent.
Color is an important factor when choosing a stain, but suitability for the surface is just as important, if not more so. Acid stains that react with the material in the concrete can be used to create a mottled effect that resembles stone, but you can't use these stains on any surface that has been previously washed with muriatic acid -- the acid removes the lime needed for the reaction. Water-based dyes and pigment stains, which are the alternative, are available in a wider range of colors and create a more uniform stain effect. If color is the only concern, choose an acid stain for earth tones and metallic blue-greens and a pigment stain or dye if you want a brighter, more vibrant color.
Stains can enhance a concrete surface, but they can't hide cracks, chips or other defects, so if your patio is heavily damaged, you may want to consider applying a skim coat of mortar before staining. If you don't do this, you need to clean the patio carefully before staining.
Sweep off leaves and other debris. Use a rake to loosen bits of debris that are stuck to the concrete.
Strip off any sealer or existing finish, using a chemical stripper, a power washer or both. This is essential -- the stain won't work if it can't penetrate.
Scrub the patio with a strong solution of trisodium phosphate and water -- 1 cup of TSP per gallon is a strong solution. Be sure to wear rubber gloves and goggles when doing this. TSP cuts through grease, dirt and anything else that might block the stain.
Rinse the patio with clear water. Wet it down again just before staining.
Mask off the side of the house with painter's tape and plastic sheeting. Cover anything else you need to protect from the stain, such as gravel walkways or brick edging. Get a 2-foot length of 1/4-inch plywood -- you can use it to protect the lawn from the spray as you stain.
The best tool for applying the acid stain -- as well as pigmented stain or dye -- is a garden sprayer. Don't use a paint sprayer -- the overspray is too hard to control. Wear protective clothing, gloves and goggles while staining.
Pour the stain mixture into the sprayer bucket. If mixing is required, use the procedure recommended on the container label. Screw the cap onto the sprayer and pump the handle to create pressure.
Wet the concrete with a garden hose -- if you haven't done so already -- and spray a single, even coat over the entire surface. Even out the stain with a paint roller if you want a uniform color distribution. If you're looking for a mottled, stonelike appearance, let the stain settle in unevenly in patches.
Let the stain dry overnight, then examine the color and spray more stain if you want it to be darker. If you want the concrete to look like marble, spray a stain with a different color.
Neutralize the acid in the stain by mixing a solution of baking soda per gallon of water and water and using it to wash the patio. Scrub the baking soda solution into the concrete with a scrub brush, then rinse with clear water.