Ospho's active ingredient to formulate iron oxide to another material is phosphoric acid. I've been using these rust reformulating products for years. I paint things like cooling towers, tanks, vehicles,and steel equipment.
One of the best iron/rust etching compounds is Martin-Senour Iron Etch. It is sold at auto body shops and is fairly expensive. It too, is phosphoric acid based.
Lately, I've been using water softener cleaner, the liquid stuff sold at most big box stores. It is inexpensive and contains a high concentration of phosphoric acid.
Besides Ospho, there are other rust formulation products that can be directly painted. Some work better than others. I've found that the additional rinsing step between applying an etchant and painting is worth the labor cost. I'm sure to get any materials off the surface that interferes with paint adhesion and, since I use scuff pads and brushes for this, it allows for better surface prep.
One further tip: Always apply a good primer before finish paint is applied. Since the general public becomes lazier as time goes forward, there are many "Primer and Paint" in one can being sold. I guess if I made and sold paint, I might make that product, also. It's a disservice to the consumer, IMO. One of the better metal primers is Sherwin-Williams Industrial DTM Primer. It's available through your local SW store. It will bond to different metals, directly, including aluminum.
Good luck with the painting!
