I spent all week restoring the 107-year old fir floor in our new house. I spent an entire day running the sander to get the old varnish off and smoothing the floor. You can still tell where someone, who knows when, had rugs down in the dining room and foyer. In spite of sanding to bare wood, the wood that would have been under the rugs is still lighter than that around it, but I'm pretty happy with the end result.
Before:
dining room
living room
foyer
After 2 coats of penetrating sealer and 2 coats of satin finish:
Before:
dining room
living room
foyer
After 2 coats of penetrating sealer and 2 coats of satin finish: