Carlton,
Sounds like you are hesitant to jump in...why not take a cleaning recommendation you are comfortable with from above and give it a shot to a small area on the back of the sign? Or, type in "porcelain sign" on your local craigslist, and buy the nastiest, hole infested porcelain sign in your area for $40 and go to town? Personally, I use Goop, its a gritless handcleaner. What Goop leaves behind, I am comfortable with. It took mother nature 80+ years to cook that sign, and I think it looks great as is. Minus a professional sign restoration, you are never going to replace the missing metal, or lost color shelving. Just my opinion, my speech is over.
Paul
