The best way with iron is like I do artillery shells. Electrolysis which
@Duane Pringle has done but the key is to get all the moisture out of the metal or it will continues to rust from the inside out. To do it right you need to boil in wax for at least 4 - 6 hours. Remove and let dry and there you go. Ive found over the years that the paint or poly only traps the moisture on the inside.