Q: I have 3 coats of satin water based poly, over 1 coat of stain, on red oak floors. The satin finish is too flat. Can I buff the floors with a 220 and apply a semigloss coat over the satin?
A: 220 would be a good place to start. You don’t want to cut through the finish and remove patches of stain. 220 should be fine enough to be safe. Whether it will be coarse enough to buff the finish you are using I don’t know. Probably it will, but if not, then you would have to go with the next grit down which is 180.