We're moving to a new home, and the first thing we are doing is the floors. We rented the orbital sander from Home Depot. The floor had a dark stain on, and as we sanded with the medium grit, previous incorrect sanding became obvious. There are these diagonal lines and ripples in several places. We were not going to stain again, but now, we might have to. We are using the coarse grit now, but it's not getting rid of the marks.

Do we need to do a better job, with a stronger sander to get rid of the marks?

Is there going to be remnant stain anyway, and should we still use oil stain and oil-based polyurethane?

If we decide to just stain it now, are there water based stains that we must use with the waterborne polyurethane?

Are water-based stain and urethanes faster-drying and better for the environment, or equal to oil-based?

What about the future? Would water-based stuff be better?

Maybe I don't know what I'm talking about.
Any enlightment anyone can provide will be highly appreciated!!!

Thanks.
JKQ