That indeed used to be a problem when minis were mostly lead, some people kept their bought and painted minis in a refrigerator because it slowed the oxidation process, I am not sure if that was correct or not but they swore by it, you could spray prime over it usually with no problem, but in some cases after several years you could have some flaking off of the paint since the paint was adhere to the oxidation (gray dust) and not the metal, I used to use Krylon it bonded well with the rust/oxidation. When that wasn't available I either washed my figures with easy green like substance, scrubbing them with a worn soft toothbrush, or at least warm soapy water it usually did the trick. I still have all led figures from back in the early seventies that are holding their paint quite well.
I sometimes miss the old lead figs, being lead they were easy to convert, a dremel circular saw blade or a good razor saw would cut through in a flash, and drilling for pinning was almost as easy as drilling and pinning plastic, in some cases you could bend the figures into the position you wanted with padded pliers. head and weapon swaps were a snap, of course spears and weapons sometimes drooped, but rarely broke. Oh well there are trade offs on everything, but I sure do miss the often less than 25 cents a figure prices.

steveb