We are refinishing our old harwood floors believed to be fir. Original owner muct have had area rugs on floor and finished wood floor around them. Now after sanding the floors there is a stain in the shape of a perfect square where there was a rug. How do I remove the stain?

Every time I've refinished floors the only successful deep stain removing process I've found is by sanding it out. I'm sure someone's is going to tell you about some bleaching process, and I'm wanting to hear it too, but for me it was an electric sander and lots of pads/belts. Fir takes stain pretty deep so I'm wondering. Good luck.