Wood and water just don't mix. Sealed and waxed floors may become stained if water sits on their surface for more than a few minutes. Here's how to repair the damage.
To remove a smoky white haze or a white spot, buff the finish with a soft cotton cloth and a very mild abrasive, such as whitening toothpaste, auto-polishing compound, or tobacco ash mixed with mineral oil, until the stain disappears (see A).
Mask off the surrounding boards with painter's masking tape.
First remove any wax or surface finish. Use fine steel wool to remove wax, and sandpaper to remove a surface finish. Depending on the size of the repair area, hand-sand with a rubber sanding block (see B) or machine-sand with a random-orbit sander. In both cases, start with 80- or even 60-grit abrasive and sand to 100-grit. Sand just up to the edges of unaffected boards.
Mix oxalic acid crystals (available at paint or hardware stores) in 1 cup (8 fl oz/250 ml) or so of hot water and stir to dissolve them. Keep adding crystals until they won't dissolve anymore.
Pour, brush or sponge the solution on the stained area. When it is completely dry, brush off the crystals and repeat the process until the stain is gone.