After stripping and degreasing as suggested, scrubbing with a strong solution of TSP in water will often bleach/lighten the darkened wood. The water will usually turn dark brown when this is done. I used to soak them in TSP but found out the hard way that if the wood is really punky the solution will dissolve the surface wood, resulting in the surface being lowered, i.e. leaving the wood below the surface of the adjoining metal (ouch!). But a good thorough-but-brief scrubbing will not harm it while helping to lighten it. Sometimes it works wonders. On the other hand I have a forend right now that I've done everthing to and it just won't lighten.
Jim
|