Though we would like to lay all the blame at the feet of the auto makers and they could do much more, even if the steel were better, the general consensus is that the weld areas are the most susceptible to rust. Joining metal with epoxy or fasteners would go along way but then, what about crash tests. That’s why when you look for areas where rust starts first, look to the welds. In the rocker panels, as in the bottom of the doors, the welds on most cars are at the bottom. That’s where most of the welds are, that’s where much of the water and salt congregates from the inside. Displace the waiter in these areas and rust can be delayed for many years and is relegated to starting on the outside where you can see, neutralize, sand and paint it. Bottom line, cars are really not made to last in the worse areas of the US more then six years, the length of most perforation warrantees.
Many years ago, the first time I removed a fender to paint on my car that was just four years old (damage) and showed no external rust what so ever, I was amazed how much rust had started on the interior. I was convinced then that we can’t sit around and wait for rust to bubble through before we do something. It was then I started listening to a good friend at work who kept cars over twenty years old that he still used for winter driving and had no rust what so ever.