Fifteen years of iron keel experience - "It''s NOT paint, but a coating!!" The first couple of years we used paint, and the results were as described by "sailingfool" - immediate rust.
Next, attempt was with a two part epoxy. Sandblast it clean. Put one coat of epoxy barrier coat, like Interlux 2000, to hold the blast. Fair the surface with epoxy and shape to perfection. Next, apply about 5 coats of the epoxy barrier coating.
Follow the instructions for preparing the barrier coat to receive your AF bottom paint. It blooms and makes adhesion difficult. Learned this after the third or fourth AF paint job!!
Next, stay away from rocks. It will last your sailing life time.