A discussion of Fertan, a rust converter, is presented. The author became interested in Fertan upon learning of its use on the Holland I, the Royal Navy's first submarine. The submarine was launched in 1901, sunk in 1913, and was raised from the bottom of the English Channel in 1981. Fertan was used in preserving the Holland 1 and in several other projects, which are described briefly. The basic chemical processes involved in corrosion of iron are described, along with traditional approaches to its treatment. When corrosion products are removed (e.g., by grinding or sand blasting), corrosion may start again immediately. Protecting surfaces with oil, grease, or paint prevents renewed corrosion as long as the protective layer is intact, and this may not be long in polluted urban environments with acidic precipitation. An additional problem is that removing all corrosion may be nearly impossible on irregular surfaces with many fissures and pits. Fertan, like other rust converters, is based on tannin derivatives, phosphoric acid, and wetting agents, and can convert unstable iron oxides or rust to a stable ferric tannate. Fertan thus does not require removal of corrosion products. The new, passive insoluble organometallic compound is as hard as the original metal. Not all aspects of the chemical reactions involved in the conversion are understood. Related products are argued to be less effective than Fertan.