Rid O'Rust Stain Remover removes rust stains caused by irrigation from most inorganic surfaces including metal siding, concrete, stucco, grout and the like. Rid O' Rust Rust Stain Remover may leave a white residue on dark surfaces such as asphalt and red brick unless it is rinsed off promptly after application. DO NOT apply to materials that contain iron oxide either in the surface pigment or as a constituent element of the material, as Rust Stain Remover may take the color out of the material. ALWAYS test a small area first, apply and then wait for the surface to dry thoroughly before continuing.