WD-40 should be safe on metal, but it will definitely get everything dirty quickly--make sure to only use a tiny amount and have a paper towel ready to clean asap. WD-40 is good for cleaning glue-type residue, like sticky stuff remaining from paper and plastic labels. I don't know what type of stain is on your case, however--if it's actual damage to the finish, WD-40 will not help.