I just went through this same thing only in a sunroom addition. I tried power tools, drills with sanding wheels, sand paper, scrapers, and chemicals. The thing that worked for me was to rent a power floor scrubber with a sanding plate. This was easier than I thought it would be and I completed in less than four hours an area 12 feet by 30 feet. Save your hands, back and hours with the proper tools.
Mari