How do you get out a dark old mildew stain on my wood floor?

I use wood bleach to get dark stains off hardwood floors, it may take repeated applications to get it light enough.... then match the floor stain

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Try washing it with Murphy's oil soap, follow the directions on the bottle, dry completely. You can also try Old English wood stain. The stain come in two finishes, you need the dark stain. If you put some stain on a rag and rub lightly the stain might be able to come up. It can also cover the area, if you spread it out a little. Put it on in a circular motion, let it dry, about 3 hrs, then wipe with paper towels until the towels have no stain on them.