Patina is a fine coating of a green-coloured oxide that forms on metals, particularly bronze and copper. It forms because of age and weather factors. If a metal planter or statue was sitting on a concrete window sill, the patina could stain the concrete. Patina stains are chemically similar to rust stains and both should be removed from concrete in the same manner. They are easiest removed when they are first caught. The longer they sit on the concrete, the more difficult removal may be.
Cut a lemon in half with a knife. Rub the lemon into the patina stain on the concrete. Allow the lemon juice to work through the concrete for five minutes.
Fill a bucket with warm water. If desired, add 1 tbsp liquid dish soap for every 2 cups of water. This won't necessarily help remove the patina, but will help remove any other grease or residue.
Dip a medium- to hard-bristled cleaning brush into the water. Scrub the concrete vigorously.
Pour vinegar over the patina stain. Allow the vinegar to work into the stain for five minutes.
Dip a medium- to hard-bristled cleaning brush into your bucket with water. Scrub the concrete vigorously.
Repeat the steps as needed to remove the patina.
If this does not remove the patina from the concrete window sill, purchase a concrete rust-removing product. Spray or apply it to the stain and then scrub the concrete clean. Rust-removing products contain harsh chemicals that can damage the siding of the house under a window sill, and should only be used as a last resort.
Muriatic acid is sometimes recommended as a concrete stain remover. However, muriatic acid actually etches and eats away at the concrete. While this can remove the stain, it ruins concrete and should be avoided.