if you use a little Barkeeper's Friend (By the cleansers in the supermarket) you can probably remove that rust without damaging the porcelain. There are other products out there that people may chime in on. Wet the sign, sprinkle some on...use your fingers to spread it around, let it sit for a few...then with either a gentle scouring pad or some 000 grade steel wool, that will come right off. Rinse...repeat as necessary... Don't over scrub. Patience is your friend. For what it is worth.