Hi !I may have a suggestion for you..... My husband and I just discovered that the floor underneath the carpet in our new home is cedar (square block wood). Just this weekend we sanded it down via trial and error and I think you'll have the most success with the following:
Rent an "Oscillating" scander (square buff sander) from Home Depot or other similar store...The machine costs $30.00 a day to rent and vibrates the sandpaper back and forth instead of chewing the floor up like a belt sander would.
Use 20 grit for the first couple of passes. Run the machine diagonally across the room in one direction, then diagonally in the other direction. Then lengthwise, then horizontally. This should total four passes over the floor.
Use 36 grit for an additional four passes.
Then a 120 screen sander to finish. Of course this formula depends on the amount of varathane you have on your floor. It worked well for ours.
We're in the process of selecting staining tonight and wrapping up the job this weekend.
Good luck with your project ! Hope that this helps :)