OSPHO is a rust-inhibiting coating not a paint. OSPHO causes iron oxide (rust) to chemically change to iron phosphate an inert, hard substance that turns the metal black. Use on new metals, for new ferrous or aluminum metals. Is effective for exterior and interior work alike. Recommended for use under oil-base primers and paints. Test a small area before using under epoxy or latex base paint. Covers approximately 600 sq. ft. per gallon. Normally requires overnight drying before topcoating. Contains phosphoric acid. Keep out of reach of children. Use adequate ventilation in confined areas, skin and eye protection should be used.