It can be a real pain sanding a roof like that. Best advice I can give is when you are doing your final blocking use a long block (at least 12" preferably 16-17") fresh paper and very little pressure. Unless you are a total hamfist, you are not going to dent it stripping the paint or lightly DA'ing it. It becomes a concern if you are having to do any filler work or final blocking. Like I said keep the pressure very, very, light and constantly change paper. If you can access the backside, as a last resort you could try putting a square of sound deadening material like Evercoat "Q" pads. Make sure the roof is how want it (no depresions, dents etc) then mount the pad to the middle of the roof are (area you are working) That should help you with stiffness.