I have found a partial solution for the classic problems with the transparent glass roof for the C5. These often have their hard, outer coating peel off. The damage is always a stripe on the passenger side running front to back.
There are 2 steps:
1.) Wet sand the affected area with 2000 grit wet-dry paper. Sand until the edges of the coating are feathered out.
2.) Polish with lamb's wool pad on an automotive polishing machine or a smaller pad in a power drill. Use a very fine compound like 3M's that is meant for hand-glazing.
While not perfect, the roof's appearance is easily ten times better than the dull blotchy stripe you started with.
Mike