If the finish of your guitar is plain wood, it's likely finished with oil..either lnnseed or tunge. What I would do, would be to buff thefinishwith very fine steel wool (oooo) going with the grain until the faded or dirty area is the same as the rest of the guitar. Then apply a new oil finish with a soft cloth. After the first coat the grain of the wood might "stand up " a bit and become a little rough. Simply repeat the procedure and you should be fine! I recommend tunge oil, by the way!