I would use a die grinder with scotch pads attachment. Or just scotch pads with manual labor from my hands, perhaps a wire brush.
and then use a Starrett metal ruler and feeler gauges to check for flatness.
if needed find a piece of glass and lay down a sheet of sand paper (4000 grit) and do some poor boy garage machining. I use a surface plate, and use those 6" "stickey" orbital sanding disc, and I can get things very flat.
Last edited by Vern_in_IL; 07/23/19 10:08 PM.