Tom:
Welcome. You're not too far from me here in Michigan.
If there is no shine now - you won't be able to polish it up. Porcelain is tough stuff. If you're dead set on a shine - finish sanding it with some 60 - 1000 grit wet-or-dry and shoot a coat of clear on the sign. That will make it shiny and protect it from further rusting.
Otherwise, a little automotive past wax can make a sign presentable.
Later . . .
Jim
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www.T-waysGraphics.com [This message has been edited by T-way (edited 01-31-2006).]