You have more than one option for staining your interior concrete floor, depending on the colors and the effects you're after. Acid stains react with the calcium hydroxide in concrete, and because they often react unevenly, you can achieve a variety if interesting, naturalistic patterns, such as marbling. Water-based stains and dyes produce more vibrant and uniform color -- especially dyes, which can be water- or solvent-based and sink deep into the pores of the concrete. Dyes and stains require the same amount of floor preparation, and you use the same techniques to apply them.
Preparing the Floor
Move all furniture out of the room. If the floor has baseboards, pry them off with a pry bar and stack them outside the room to prevent them from getting stain on them. Vacuum the floor and wash it with all-purpose floor cleaner and warm water in preparation for stripping and sanding. If the floor is new, and you aren't going to sand it, wash it instead with strong detergent -- such as trisodium phosphate or a substitute. An appropriate solution is 1/2 cup of TSP per gallon of water.
If the floor has a previous coat of sealant, you have to strip it, or the stain won't penetrate. Use a concrete floor stripping product, available at any building center. Strippers are similar to paint strippers; use a methylene-chloride-based one if you're in a hurry or a soy- or citrus-based product if you're concerned about yourself and the environment. Spread the stripper with a paint roller, give it time to work, then scrape it off with a floor scraper and dispose of it responsibly. Wash the floor down with water after stripping.
Sanding the floor is a mechanical alternative to stripping, and you should do it if the floor is smooth and polished. Use a floor buffer and a 150-grit sanding screen to lightly etch the surface of the floor. Sanding removes a small amount of material from the surface, so don't overdo it.
Use a shop vacuum to remove all traces of sanding dust, then wash the floor again with TSP and water. Rinse with clear water and allow the floor to dry completely.
While the floor is drying, mask off anything at floor level that you want to protect from the stain, including the walls, posts and cabinets. Masking paper should extend at least 12 inches up the sides of the walls or cabinets.
Applying the Stain or Dye
Mix the stain or dye with water or an appropriate solvent if needed and as recommended in the instructions. Put the stain into a handheld airless spray gun and spray a small section of the floor in an inconspicuous place to get an idea of how much you need to spray to get the color effect you want.
Moving the sprayer in a back-and-forth or circular motion, start spraying in the corner of the room farthest from the door and work your way toward the door. Get a helper to walk behind you with a broom to even out the stain and work it into the concrete. Avoid leaving any puddles.
Let the stain dry, then spray again if you want to layer different colors to get mottled effects.
If you use an acid stain, you need to neutralize if after it has finished reacting. Do this by spraying the floor with a solution consisting of 1 part ammonia to 4 parts water. Use a different spray cup and a different tip or, if you prefer, a garden sprayer. You can also neutralize with baking soda in the same proportions. If you use water stain or dye, wash any residue off the floor with water.
Spread concrete floor sealer with a paint roller and a paintbrush after the stain or dye has completely dried. Let the sealer dry for the recommended time -- usually 24 hours -- before walking on the floor.