Am I right in thinking you are using the old buffer as an orbital sander? Where are you getting sand sheets big enough for it?
I cut the sandpaper sheets in a circle for that one. I also use a pad where the sand paper adheres on one side while the other side of the pad is pressured against the bristles. It's very difficult to manage that's why I went ahead and rented the sander.
Got to go back now, I am doing the 120 grid now which means I am almost done with one room (the green one) then I am heading for the other room (the blue one).