I use Mothers Mag Wheel cleaner. It works great on porcelain signs and even works well on painted surfaces like oil cans, but you have to be careful anytime you are using a chemical on a painted surface! This stuff is really mild but it can take some paint off in some instances. Like I said, I can pretty well tell if a can has weak paint or not, and I always try it in an inconspicuous area first.
I have always had really good luck with this stuff because it is so mild, yet it seems to remove most petroleum based crud and marks. I would recommend trying it on some old can that really isn't woth anything and get used to the product before you try it out on the Holy Grail! LOL
