I know this has been asked, but most threads trail off into how to "preserve" a rosewood fretboard. My concern is cleaning the gunk off of one. I bought a used guitar where the grime is caked around the frets and from what I see it eventually gets into and ruins the strings. What cleaning fluid and applicator would you use to clean a rosewood fretboard? I have read that lemon oil is okay for cleaning with a scrubby dishrag. What about a light toothbrush? For the preserving part I have read a light mineral oil (food grade) or bore oil is best. What do the pros use? But my main concern is cleaning - I know some people say "just use a cloth" or "an old pick to scrape it off," but my hope is that I can restore the rosewood to like new condition.