Ospho is excellent rust converter, however, it's expensive. You can buy an equally effective product for half the price at Home Depot or Lowe's. It's called "Phosphoric Etch and Prep" It's sold under the "Klean-Strip" label, in the paint department.
Apply it and wait for the rust to start turning black, then rinse off as much of the residue as you can, while it's still wet. If you wait for it to dry, you'll have to wire brush all the white powder off everything, and if any gets left on there, it interferes with the paint adhesion. Once it's rinsed off, let it dry thoroughly, and paint with a good oil-based primer, followed up by an oil based paint.