I just removed the glass on an 84 to prepare for paint. While there was no rust apparent beyond the gasket when I started, there was some rust underneath the gasket. The reason rust gets started in there is that water gets under the seal and finds its way in between the inner and outer metal plates that form the hatch where the metal is not protected. The panels are just sandwiched together around the window and tack welded every few inches. If the metal is entirely gone under the seal in spots where its rusted, you'll need to cut and weld in new patches of good metal. I wouldn't do fiberglass here because I'd be afraid it would crack while putting the glass back in. However, you need to prevent the rust from coming back in other spots as its all in between the sandwiched plates along the entire bottom of the glass. I treated this 84's hatch with liberal doses of Ospho (phosporic acid) injected between the sandwich plates with a syringe to kill the rust. Next, I ran a bead of JB Weld epoxy all along the sandwich edge to seal it so water can never get in between them again. Lastly, when I put the glass back in after painting, I'll make sure its sealed well with some caulking in addition to the gasket. Next time the glass has to come out, the gasket will have to be cut, but it damn sure won't have any more rust.
Phil D.
Phil D.