A method for removing rust, scales or spots formed on metallic surfaces is described. The method involves contacting the metallic surface with a cultured suspension of chemoautrophic ferrobacillus at a temperature in the range from 20 to 45°C. The conditions should be aerobic, and the ferrobacillus utilizes carbon dioxide, inorganic nitrogen compounds and takes the necessary energy from the oxidation energy of sulfur and/or iron. The micro-organisms used are <i>ferrobacillus ferroxidans or ferrobacillussulpho-oxidans. With conventionally used phosphoric acid solution the rust removability decreases with the number of treatments it is used for. This is not the case with the micro-organism culture due to bacterial growth during removal of rust. -- AATA