Here's a quick trick for fixing up rust buckets!
In one case, (my mothers car) the rust was so bad (and old Falcon) that the trunk wells were no longer... huge 1 foot holes and the outside fenders from the top of the wheel down was space... you could see what was in the trunk without lifting the lid.... ditto for most of the car, rockers showed the frame etc.
I suppose I could have spent days fabricating metal.. I told here to junk it, but it was still on the road and she didn't have a lot of bucks. The other case wasn't quite so bad.. a Honda Civic with rocker and fender problems... I used the following method and it works like a charm and in 2 hours no less.
You go get yourself some polyurethane foam, this is available from most boating supply shops and fiberglass outfitters by the gallon/quart. You get two components 50/50 and you mix them together with a drill (high speed because this happens very quickly) the consistency is like honey at first, but expands to 10-50 times it's size (it's plastic foam) and you pour it into 'prefabbed cardboard form shapes that you've taped to the car. The forms you make must contain the foam as it's expanding and must roughly be the spaes you need. 10 minutes later... Volia plastic patches. Next using something like a surfoam rasp (those metal scrapping tools with tiny holes) you shape it and then slide a thin coat of bondo (fiberglass if the part is bigger than 1 foot although I've never bothered) on top and you have something that is close to paint and 1 hour has passed.
Best of all, it's great insulation, seals out further water damage and usually fills the rockers. I know RFDers can appreciate how low one can go...