Sharpie is alcohol type based ink, that is the ink Sharpie uses is soluble in alcohol...so if it's Sharpie, alcohol may remove the ink. But it's unlikely to have much affect upon any discoloration that the ink may have wrought on the top/clear coat. Naphthalene is also good, but again with the same caveat. However... the bright side is that if the clear coat is poly, any discoloration will probably not have penetrated to the color coat. So it is possible that you could chemically remove the clear coat entirely to eliminate the graffiti. Then reshoot a clear coat of your choice and bingo...new guitar that technically wouldn't be a refin!