Is this a keel-stepped mast? When we have these problems around here, they're usually in the form of an aluminium step "welded" into an aluminium mast heel, resting atop a corroded steel support under the cabin sole.
Before isolating, gelling, etc., we always seriously clean all the metal surfaces, even using a little muriatic acid to speed things along, finishing with a good round of punishment with a non-ferrous wire wheel; you really want to knock all the gunk out of the pits and see what you have to work with. Depending on the level of corrosion, by all means go ahead and get rough with the chipping hammer; better to expose a weakness now than later, during use!
After that, we use several coats of zinc-chromate primer on the aluminium, and Ospho followed by red oxide primer on steel. Stainless naturally benefits from close polishing and re-passivating. i think that solid surface prep prior to isolation attempts is key.
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