I have a 60 year old house and the original hardwood floors have been covered for at least 25 years and the slats are cupped with the edges of each slat being higher than the center of the slat. What is the best way to sand these floors smooth so that i may put either another layer of hard woods or vinyl slats over the top of these existing floors?
Worst case scenario: remove the flooring.
Medium damage throughout: Lay down 3/8" plywood over it, first replacing heavily damaged areas.
Every board is slightly cupped: Have it professionally sanded and refinished. Upright floor sanders can be rented but they are not for novices. A random orbit sander would get it done... eventually.
You might consider removing the slats and replacing them with sheets of plywood - it's likely to be less work than anything else you will do.
You could use steam to soften the wood, and nail the edges, but that's a ton of work.
You could try a huge floor sander, but they are not made for removing that much wood, and you will be left with very weak slats.
I would remove them, it's less work than the alternatives.
Cupped floorboards are generally caused by water damage. If the cupping is slight, you can sand it with a commercial sander.
If they are too far gone, then it might be better to remove the wood and replace it instead of going over the top of it. If you add another layer to it, then it would throw off the height of the thresholds, doors, moldings, etc.