I believe the sunroof panel is made of two pieces of stamped sheet metal. They should be spot welded around the outside lip and glued together in the middle. You'd have to grind/drill out the spot welds and separate the glue. once you have the two panels apart, you can sand them or sandblast them and paint them with POR-15. Reassemble them as before, and voila... they'll never rust again. It's a lot of work, but it's the only way of guaranteeing the rust won't be a problem again...at least not for a long time.