I work for a US Navy museum with outdoor, metal artifacts. I would first recommend that you take the rust down to one layer than use a rust converter to stabilize it (the converter is black). If you do not desire a black finish, bring it down to bare metal. Renaissance Wax is a great inert product. You want to work in small areas on a hot, dry day. Heat the metal sculpture, and then apply the wax. The warmth of the structure will melt the wax allowing longer protection. Do not heat the wax directly because it is flammable. I also highly recommend the Care of Metals Artifacts class at the Campbell Center http://www.campbellcenter.org/. It is taught by two amazing Metals Conservators. Best of luck!