with that much rust, I would seriously consider trying to find a less rusted body to start with. Make sure you use some type of rust converter. after each weld I would use a spray galv coating and after it dries, use a good primer. on my vehicle I used red oxide primer and eastwood frame coating. after that I used a truck bed liner on all my frame, underbody and even inside the panels in the vehicle. they make a coating from eastwood that has a tube you can seal the inside of the frame. it isnt cheap, but at least use something to spray inside the frame. even rustoleum would be ok. just make sure u seal every weld. but really use the galv spray or a zinc primer. I weld for a living and wouldnt ever use just a regular primer for the welds. they will eventually rust if not sealed good.