How do you prepare and stain an existing interior concrete floor?

Apply muriatic acid to dry surface (good ventilation a must). This removes stains and oils. (Note: if you plan on using an acid stain, DO NOT clean with muriatic acid, since this will make acid staining impossible). Rinse and dry. Apply sodium hydroxide (10% solution) to neutralize remaining acid. Rinse and dry. Ready for your choice of floor treatment. Recommend a urethane sealant first, to prevent moisture absorbed in concrete from coming up under the coating you finish with.