I get patches of grey on my fretboard, which im guessing is skin, and also i get bits of fluff and a build up on gammy stuff either side of the mettle strips on the frets. I have treid taking off my strings and cleaning it with water and a bit of soap in it and tried to scrub the mettle bits with an old tooth brush to get rid of the bits there. But to no prevail :( There is still bits either side of the mettle bits, for a while the grey patches of skin went but they soon built up again.

Does anybody have a technique that works well or know anything you can use on your fret board that wont discolour the wood or anything?