Well here is what works for me. I picked this sign up a while back as a good sign but only one side useable. It is the long Richfield and someone had taken great efforts to paint GARAGE on the backside. I got it pretty cheap because only one side was good according to the seller. Here is what I did. I have included pictures.Here is all you have to do.
Lay sign out and apply aviation paint stripper with an old throw away brush.
Let it sit and watch it bubble up and crinkle.About 10 to 30 minutes does it. Then scrap it off. I haven't found too much that can hurt the porcelain. Rinse it off with water and dry.
Then I sometimes use a light cut rubbing compound by 3M and remove any stains or imperfections. A heavy cut can be used for stubborn marks. Then I follow up with a good liquid polish. There you have it, and the side shown is the end product. It turned out better than the original good side. Have fun. Ted