Flagstone is a term used for a variety of types of stone that are suitable for paving or flooring. It is slightly porous and is usually sealed when used indoors, but outdoor flagstone is allowed to weather. Flagstone is exposed to many potential staining compounds whether it is inside or outside the home. Exterior flagstone is exposed to more dirt and debris than indoor stone, but either one can suddenly present you with spots and smears. These are easiest to remove if you know what the stain is, but if you don't, you may have to experiment with a couple of different cleaning methods to get out the staining.
Use a pressure washer on outdoor flagstone to blast away the stain. This will work on many types of stains but not oily, rust or chemical marks. The pressure washer can be used with straight water or a soap mixture to remove the stain.
Make a paste of a degreaser dish soap and rub it into the stain. Use a scrub brush to really get into the grooves and pits of the stones. Rinse the area thoroughly and re-scrub if the stain is still there. This works on oily stains.
Apply a stone cleaner and brightener according to the package directions. This type of product will act almost as a bleach and pull out unsightly discoloration. It also rejuvenates the stone, especially if it is old and unsealed. It is perfect for application prior to sealing flagstone.
Make a dilution of the outdoor mildew cleaner as the manufacturer recommends. You can mix it in the spray bottle with the required amount of water and spray it on the stains if they are organic. Most products require the chemical to sit on the stain for 30 seconds, but it may vary with product. Agitating the stain with a scrub brush will help remove it. After the correct amount of time has elapsed, rinse the stain away.
Soak a rag in vinegar and apply to the stain. You may have to scrub a bit. Vinegar contains acetic acid, which will break down numerous types of stains. It is also nontoxic, making it ideal for use in the home where pets and children may range.
Seal the flagstone when you have successfully removed the stain. If staining occurs in the future, you won't have to work so hard to remove them. Sealing will help keep the flagstone from weathering and chipping on the edges so quickly.