A buddy gave me a set of dies he had that were rusty he had replaced with new ones . I used a drill and a wooden dowel with a thin saw kerf in the end of it to hold a small piece of 3m scotch bright (the grey extra fine) to hone the inside spraying with pb blaster. Finished that off with a piece of t-shirt and mag wheel polish on the dowel and the inside looked like new. This works good if you have let the rust get ahead of you. Still using those dies. Ant nothing much better than free….
