holy crap, wish i'd seen this thread sooner. per zak smith (former mod here and owner of thunderbeast), i have used CLR (calcium lime rust) to clean all my cans for about 10 years now and it works as well as anything and is relatively safe.
first, weigh your can on a scale
take a plastic water bottle and cut the top off. pour about an inch or two of CLR in it. pour 1-2x that much water in.
now drop your can in. wait til fizzing stops. slosh it around a bit. let it sit a while. rinse, repeat. (for me, rinsing usually involves blowing it out with water and then compressed air too)
go 3-4 cycles of that, with some shaking and swishing. then dry and weigh it again. report how much carbon you removed