This seems to be a great solution to convert a rusty piece of metal into a paintable surface.
1 Gallon $28 Ace hardware (2020)
HOW SKYCO OSPHO WORKS
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.
Mix half & half with plain tap water. Add about 1/4 cup liq dish washing det like DAWN ( which retards the instant runoff of the just plain Ospho and acts as a surfactant to hold the Ospho on the surface. Take a nylon cheap bristle paint brush and paint the rusty stained area starting at lower and moving upward. By the time you finish wetting the area to be treated with the Ospho the first area treated will already be front tooth white.
Use also for sinks, Lavatories, and commodes to eradicate rust stains and mildew and grime.
You can also use a quart bottle pump handle squeeze sprayer filled with the mixture to spray your shower walls without being affected by the fumes.