Hello Jordan
I have had a lot of these and have tried cleaning them in various conditions, with various cleaners. They were originally made as throw aways or re-fillers with no plans for longevity. They certainly were never intended to be collectibles. The quality of paints was minimal and has become very vulnerable with age. The only consistent method posted here seems to be a mild solution of Dove Dish Soap and water. I can attest to ruining some rocker cans with Whitewall Cleaner or Engine Degreaser or strong solvents. That is why nice cans demand a premium price. You cannot resurrect the dead, make a silk purse from a sow's ear, or clean a rocker can with decades of grime absorbed into the paint. Sorry.
Ed.