or whenever it stops fizzing,but dont let the coke dry, so keep adding coke to keep it wet. Then power wash the coke away. The stain should turn kinda gray at this point. Then mix 1 cup AMMONIA FREE powdered laundry detergent, 1 cup bleach, and 1 gallon of hot water and scrub the area with this mixture until the stain is gone. Make sure the detergent is AMMONIA FREE, or you could end up killing yourself my making homemade mustard gas.