A friend of mine is fixing up his rental unit. There was carpeting over oak floors and he is trying to restore the hardwood floors. Problem: Earlier tennant had a dog that urinated on the carpet and thus stained the oak under it. The stain has penetrated the wood very deeply leaving dark splotched in several places. He has tried sanding and of course that only affected the surface. He has tried peroxide and household bleech. Again no positive results. Is there any other approach he can take to remove these stains save ripping the floor out or covering it again with a carpet?