whatever advice you get on the internet that says you "can't" get them clean enough, think about this...
i worked as an aircraft mechanic for 22 years, during that time i had to clean engine oil, hydraulic fluid, jet fuel, and toilette waste water out of composite panels before repairing them.. replacing sometimes is NOT an option...
before somebody tells you it can't be done, it doesn't cost you anything to try. the question is, are you willing to do the work to get them clean enough to coat them... sometimes it becomes a matter of "is it worth the cost in time and materials to do so."
like i mentioned in my previous post, i have heard of people boiling them, in fact one of my best customers did so and used powder coat for a bunch of bullets before he asked me if i could size them and not lube them, he said he learned about it from other people that were doing the same thing with cast bullets they got from many different vendors and it worked.