To clean outdoor flagstone covered with moderate dirt, use a pressure cleaner. Flagstone that is very dirty may require the use of a chemical cleaner. Indoor flagstone can be cleaned using a mop.
In most cases, the abrasion created by the pressure cleaner will remove dirt and debris from the flagstone outside. Even if the pressure washer does not work completely, it is still the first step people must take before using chemical cleaner.
Chemicals such as Muriatic acid work effectively to clean flagstone as well. However, acid is very dangerous to work with and should never be used indoors. Before using the acid, it should be diluted with water until the acid is about 30 percent of the total solution. Gloves and eye protection are essential when using the acid. Apply it to the floor following the manufacturer's instructions and rinse thoroughly.
Another option is to use a bleach and water solution to clean the floor. Mix the bleach in a bucket with the water, and apply it using a mop or brush. Rinse thoroughly and allow the area to dry for 24 hours. Bleach should only be used in a well-ventilated area. After cleaning the flagstone, it is a good idea to apply a sealer to the surface to help prevent more dirt from building up.