I would take a picture of the signature before you do anything .. then you'd still have a "copy" of it. I think the clear polish would work .. I don't know if it would help with fading though. Try emailing or calling Sharpie and see if they have any suggestions.
You could cover the signature with a piece of clear contact.This would protect it from rubbing or scratching off, but don't know if it would stop it from fading or not.
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