When applied to rusted surfaces, it resists/retards rust in chemical change on drying to a tough, hard surface ready for priming. A paint job will last longer after an application of OSPHO because subsequent paint coating securely attaches itself so that moisture and oxygen normally do not attack the metal. OSPHO is water-thin, can cover a larger area than paint, and goes on easily. It is equally effective for exterior and interior work alike. SKYCO OSPHO: a balanced formula of Phosphoric, Dichromate, Wetting Agents and Extenders . . . compounded exclusively by the Skybryte Company since 1947 ... recommended by paint manufacturers.