Primer doesn't seal the metal from moisture. Body shop told me to put some enamel, spray bomb Krylon, over the repair, then sand it off when getting ready for paint. Welding is expensive. My 66 was fiberglassed in 02, 08 I had bubbles behind the doors and other places, lots of places, car was far worse than I knew. But, sealing up the rust, spray undercoat or por 15 into the inside of doors or where repairs are and just stop the rust. You can do a lot of body work by hand with some sanding blocks and get the car straighter. I worked at a body shop for a few months this year and learned a ton, and I painted a bunch of cars in the past. Find a good guy at a local shop and volunteer some time cleaning for some training, even on their cars. or watch the videos bear always talks about, paintafication or whatever. Autozone, get a gallon of bondo, follow mixing instructions, smear it on, sand it off, when it's flat, throw some paint on it.