The old floor had an overlay and tile, which I have ripped up. Nails from overlay (tile underlay) have left stains in hardwood. We are about to sand; any suggestions as to how to remove stains before sanding; otherwise, I will have a lot of dark spots at each of the former nail holes. (Must be rust stains??)

Sanding should remove the majority of the stains. What's left should not be noticible.

Be very carefull of wood fillers, I used a product by Minwax that ended up looking like Bondo on my floors.