I just had a new set of stairs built and put in by my contractor and now I'm staining it myself. The treads are unfinished red oak, so i sanded it, applied pre-stain conditioner and when I went to apply the Minwax dark walnut oil based stain, there was one spot on the treads where the stain just would not penetrate. The rest of that particular tread took the stain fine but just that one spot where it didn't and still has that unfinished wood look. What would cause that and how do I get it that spot to absorb the stain? Do I need to re-sand that spot and clean it with something?