My wife and I go back to the internet to try to figure out exactly what this job is going to take. Hind-sight would have told us to use cabinet-grade plywood on the floor, since we were going to be painting it, but we didn't know we were going to tackle this project until well into our house's construction. Our builder insisted that no matter how much we painted and primed, we weren't going to be able to cover up the seams and imperfections that come with standard grade OSB plywood. But we already committed to the cause, so let's make the best of it!
So the first step, sanding. We started out in my son's room because he wanted a 'boat deck' theme. We figured this would be the most forgiving of any imperfections, since it's supposed to look 'rough' anyway. All 4 of us got down on our hands and knees, with little pieces of sandpaper in our hands, and started scrubbing. 10 minutes go by...20 minutes...a half hour...still scrubbing. While the floor was getting smoother, it definitely was not turning out how we wanted :( There was just no way of getting enough force to effectively level out that kind of board.