Remove general soil and grit with detergent and water. With steel wool, or sandpaper, or wire brush, remove all loose, peeled paint and rust. Wipe surface using cloth dampened with turpentine, or vinegar.
No paint will stick to a greasy or wet surface. First coat should be a "metal primer" or "rust retardant" type paint. Second coat can be of enamel or paint suitable for place item is to be used, usually outdoor enamel. When painted surface is completely dry (a couple days) apply protective finish as if item were new.
Even small chips or scratches should be touched up immediately with primer followed by matching paint to prevent rust forming.