I have a Benelli Montefeltro 20 bore semi-auto. It has a short stock to enable my wife to shoot it and when I use it I use a rubber Pad extension boot. Like a fool I left the boot on for a few weeks. and when I took it off the stock where the boot had been had turned black!. I do not know how deep or how to get rid of it. I think that the stock is varnished but I am not sure. I know that I sound like a fool but can anybody help?