I'm trying to finish wood steps that were carpeted. After sanding them I've noticed they are discolored where they were carpeted compared to the small area that was exposed for the banister spindles. Also discolored around scratch mark apparently from where carpet was cut. I've tried oxalic acid to bleach it. It only worked for the black marks from the tack strip nails not for the discolored area. How can I even out the natural wood color?