On the interior of our boat, one of our bulkheads had pictures in frames hung by the previous owner. After many years, this has left a clear outline of where the picture was hung. I'm not sure if the resulting shadow is from a build up of grime/dust/oils over the years in the finish, or form UV exposure, but I'm here to determine what the best way to even out the finish again without having to remove the old varnish.