I worked in a shop that had wooded block floors for a number of years. The were end grain up and set in tar over concrete. When they replaced or need to be resurfaced due to build up of oil and other junk they had a machine they brought in that had fine teeth in some kind of rotating roller. This would take a thin layer of material of the surface without loading up like sandpaper would. This was a course finish with good traction. It was very easy to work on. The other thing we used to do was lay a double sheet of 3/4 plywood on the floor over the concrete in the work areas to make it easier on the back and feet.