Serious rust I think needs a serious solution..
I'd have it stripped(media blasted)Depending on what's left after that and your level of ambition, there are a few ways to fix. Adding new metal is the top choice however "fiberglass" is great for areas where water is a problem since it is water resistant..plus you can mold it with your fingers and do a desent job retaining the proper contour..The glass can be topcoated with filler for a transparent repair.
Good luck
