NO!!! Never ever consider varnishing or coating any signature with any kind of protective coating. Why? well first, that protective coating may well react with the signature dependant on what type of pen the signature was signed with, and so you might well end up with a very messy signature.

The varnish or whatever you coat it with is also likely to change colour over time and become cloudy or darker, so it will become very obvious that you have painted over it!

The only way to protect the signature from fading, and i am assuming that you would not be stupid enough to play the guitar, is to keep it out of the light and only bring out for display now and again when you want to show it off.

There are UV plastics available, and you can speak with your framer about this, but they may be able to supply a piece to put over the signature.