We started sanding the old wood floor with a 60-grit drum sander. We did sand twice with 60 grit, but we can see uneven sanded floors.
Should we sand with 36-grit or redo with 60-grit again?
The old drum sanders can have a tendency to dig in if you pause. Never have the motor running and the drum in contact with the floor when the sander is not im motion across the floor. The only way I know to actually fix it one that happens is go over the floor again and try to even it out. But you might first want to try splashing some mineral spirits across the area, to see what it will look like after being finished; the difference may not be obvious when wetted.
As a beginner, I chose to throw money at the problem. There is a newer sander design which uses four sanding disks held parallel which each other rather than a drum. This distributes the tool's weight over a much larger area, reducing the risk of it noticably sanding deeper in some places than others; it's more expensive to rent and operate but more beginner-friendly. But my lady had complete success using the old drum machines, so this was more a matter of my being able to afford a bit of applied paranoia than anything else.