If you can remove the staircase and send it out to be sandblasted, then primed and powder-coated, that's best. If it can't be removed, you can sandblast it in place by putting up plastic to protect the house, plants, etc., around it. If that's not practical, use a grinder (still put up plastic). After you've gotten off all the old paint and there's nothing loose, you can use a product called Corroseal (available at Home Depot and others) ... it's even better than Ospho at converting rust chemically. Either of these products needs to dry at least overnight (so pick a non-rainy day). Then apply a great primer, followed by 2 or 3 coats of top paint, and a final sealer. For these paint and top coats I like 2-part products that are made for the auto industry (go to an auto paint supply store for advice). They should be applied with a sprayer (a Wagner, or sprayer as part of a compressor system) for best results.