JDW,
I just prep with Birchwood (or other) blueing/rust remover, scrubbing with steelwool and a small wirebrush which gets all the oxidation off in the pits. If you have access to a fine sandblast or glassbeadblast cabinet, this will work as well. Grinding it out with a burr would be a way to go if you have trouble with finding any of the above.