What’s the best way to remove a polyurethane finish that was placed on my slate floor?

Unfortunately, most polyurethane finishes require strong solvent-based chemicals for removal. Methylene-chloride strippers as well as a number of safe strippers are available, but be forewarned: removing polyurethane is a messy job. I know several professionals who refuse to do it! Once the urethane is removed recoat the floor with a water based acrylic finish.

TIP If the polyurethane is not peeling it may be possible to remove a very thin layer of the urethane by using dry #3 steel wool. Run the steel wool over the floor, pick up the dust and re-coat with an acrylic sealer.