I purchased a new maple neck Stingray 5 three months ago. I changed the strings last night and noticed some staining(darkening)on the fret board between the strings and slightly on the back of the neck. I always was my hands before playing, and I up to this point I've used Fast fret before and after playing. This is my first bass without a "sealed neck", all of my Fenders have sealed necks. My question, Is this normal? Is it the Fast Fret? What could I use to clean it? BTW, this is one great sounding bass!