Peachy the easiest way would be to take a strip of wood that just fits the groove and glue some sandpaper to the end. Welcome to the forum by the way. Trying to get into the lips on either side is a lot trickier. Do you need to do that too?
bleach it... but experiment...Thanks! This paneling has several grooves, so I might not be able to do what I need to do. It has been stained, and I want to stain it a lighter color.