I did this early summer on my Catalina 22 swing keel (cast iron): Sandblasted to bare metal, 3 coats Interlux Interprotect 2000e, then bottom paint. Dove the boat after 6 months to clean and looks like it did when I finished. Time will tell. The Interlux was recommended by several local marine hardware places.
My understanding is rust begins immediately, so time is of the essence after sandblasting, as to not encapsulate the rust...