The majority of electric and acoustic guitars have an unfinished rosewood or ebony fingerboard. Normally, a quick wipe of the fingerboard after each use is enough to keep it clean, but for guitar players who gig a lot or do a lot of session work, a more thorough cleaning may be necessary. It’s also a smart idea to clean the fingerboard of any used instrument you may purchase.
Never use anything on your fingerboard that’s not specifically formulated for this, including furniture cleaners, which can actually damage the wood over time. Typically, a deep cleaning using a lemon oil-based fingerboard cleaner is all that’s required. First, remove the strings, then apply a small amount of oil to a soft cloth and rub it in. Leave the oil to penetrate for a few minutes, then rub off with a clean, soft cloth. If there is grime along the fret edges, apply oil to a piece of cloth wrapped around the edge of a pick (or a fingernail) and use this to gradually get rid of the buildup. For dry fingerboards on guitars used a lot in heated environments, a little lemon oil cleaner will help moisturize the fingerboard and keep it from delaminating.
Some manufacturers recommend not using lemon oil-based cleaners. This is because if left on too long, the oil can soften the wood too much. However, by using it sparingly and not leaving it to soak in, you should be fine. If your guitar is under warranty and the manufacturer recommends that only certain types of cleaners be used that they judge to be safe, you should heed that advice during the warranty period.
Maple fingerboards almost always have a finish that’s just like the guitar’s body. In this case, follow the manufacturer’s instructions for cleaning the instrument’s finish.