I use a product put out by "Johnson Wax" and it is called "Spitfire" power cleaner. It is already mixed (you can dilute it if necessary) and it does a great job on signs (especially porcelain) and cans. "Spitfire" is not available at your local Walmart, you have to pick it up at a janitorial supply house.It comes in a red & white spray bottle.
I have used mineral spirits too, you need to be careful and use it first on a test area to be certain it doesn't take the paint off.