i fixed that on my '91
you are definately gonna want to pull the interior panel and the rear washer tank and have a look in behind there. The best thing to do would be cut out the rust and strip back the paint around it so you know you are into good solid metal. Weld in a patch that you took your time to fit first. I used expandable foam to back fill the area where road gunk gets kicked up and trapped in there. It is a result of the gas filler channel being cut into that wheel well area. Use body filler on the front side and sand it out for paint. Use your hand and not you eyes to tell you when you have sanded it smooth. I can't stress how much easier your life will be if you really take your time to shape your patch piece as close to the original contours as possible. When you think you have got something pretty good, see if you can make it just a little bit better.
once you get to welding have a buddy on standby with a shot of shop air or even a spray bottle to put out any unexpected flare ups. If you can get the filler neck out of the way and remove the plastic "cup" thats behind the gas door, you won't have as many problems melting/burning that plastic and it might make it easy to work with your patch piece.
and it won't be long before we start talking "Bondo" and MONMIX tells you Bondo is a brand, and what you really want to use is a filler called Evercoat. I'll back him up in saying this stuff really is better than the bondo brand. I have used both and although i am only a hobbiest, the stuff really seems to have less air in it or something. really helps get you a nice smooth repair. Bondo will work fine and is tested and true in the eyes of the hobbiest, but if you can get that other stuff.......its the way to go
But where did you get that new quarter panel??? Nobody seems to sell them anymore. I've got a 94 sport.
