The conventional way as you know is cutting and welding panels, or replacing panels. This is very expensive, and to be quite honest with you, may not be a good choice because if you have a lot of rust, the cost of the repair will be more than what the car is worth.

But anyway, sanding it down till you see metal, then apply the anti rust protecting coats should do well. But try to get as much or all the rust off if you can before apply the anti rust compound.

My 84 300D Turbo, which has 110,000 original miles, and rarely driven is rusting. As usual the bottom end of the car around the rear wheelwell. Another place I noticed it, is in the front of the car, under there is a metal spoiler like, and the hindges might rust. You have to crawl under the car or get it in the air to see it though. Hope this helps you some...