Restoring 100 year old wooden floors

This is the first of a series of videos we made last November. Because of health issues I'm only getting around to editing and uploading them now.
We replaced the old flooring with solid oak on the first story that we renovated, however on the third floor we decided to keep the original 100-year-old pine plank flooring as it was solid, but the surface was severely damaged. It had been covered with a layer of cement leveling compound and then with 1" chipboard and finally PVC tiles. We removed the PVC, chipboard and as much of the leveling compound as possible and then sanded the boards with a 16-inch belt sander using #16-grit belts. As seen in the next videos we then sanded with 24, 40, 60, 80, 100 and finally 120-grit to get a smooth surface that can be oiled to a sheen.