After seeing the pictures of your sign I would definitely try to clean it a little more. I'm a Skelly collector myself and have that exact sign without the frame. Yours has good color and very few chips. The metal frame has contributed to a lot of the rusty stains. I would take it out of the frame and clean it and media blast and paint the frame if it were mine. There is a product that I use to clean porcelain that has rust contamination on it like yours and it works well without scrubbing too much. It is called Iron-X and is not harmful to paint and especially porcelain. I buy mine on eBay but I think it can be ordered other places. Others on here have used it on signs, face plates, and other parts with good success. You might give it a try. Nice find, you may be a future Skelly collector in the making.....GB