WD-40, oil out of can, K-Mart hand cleaner W/O PUMAS. I have even used Paint Stripper applied to a rag [NEVER APPLY DIRECTLY on can], then slowly wiped off spray paint.
ALWAYS start on worst side & slowly work to better/best side.

Reproduction/Fantasy Cans
http://i124.photobucket.com/albums/p2/oldegaspump/AllCansMASTERTHUMBNAIL_zps4bfdbbc7.jpg
Rnd Can Re-lids