I have a 45-year old ¾” thick hardwood floor (~ 350 S. F.) and need to bring it back to life. I was going to re-finish it but due to time/dust/etc., I started thinking about installing (nailing) a new hardwood floor on top of the existing one which would give the 45-year old floor more stability. I am also looking into floating hardwood floors. Has anyone done this before (i.e. nailed a new hardwood floor on top of the old one)? I appreciate for any recommendation (e.g. do I need any barrier between the two hardwood floors? Do I need to prepare the existing floor (sand it to make it more flat)?).

Thanks a lot for any advice,

You can nail new flooring to the old but not in the same direction. You will need to lay the new flooring at a 45 to 90 degree angle to the old. Unless you lay 1/2" plywood over the old flooring. Then you could run the new floor in the same direction. If you install a floating floor, just lay the foam pad and install the new floor in any direction you want. If you nail a new floor to the old, lay some 15 lb. asphalt felt down first. Or Aquabar "B" paper. I will say, you won't be saving much time over refinishing the floor. Plus new flooring costs more than a refinish does.