When I remodeled the old house, I redid the hardwood floors. There were stains embedded into the wood from the carpet pads. They were usually isolated is spots. One room i had to melt off the old foam. After light sanding with the Varathane sanding machine, the spots were still visible in to some extent. After experimenting, I found a way to get rid of the spots. i used Oxi-Clean. 2 or 3 spoonfulls into about a half to one gallon mixture. After applying one coat, wait and apply more coats as necessary, and the fluid may have to be rejunivated. You cannot see the final results until after it dries. It reacts differently with different woods. On the main floor I did the prep. After applying 3 coats of polyurethane, the floor looked terrible. Actually turned blackish in spots. I was disappointed or at least for a day or so. the wood turned beautifull after a day or two to three days, and it continued to lighten up. Other floors didn't turn black at first like the main floor, so I think the preparation and wood types can vary the process. I just wanted to possibly recommend trying oxi-clean on unfinished outside wood, but it has to be soaked on, and not pressure washed, or at least thats how i view it. The drying process is essential to the cleaning process. Several wet-dry treatments will provide the most action.