That's why I'm at a conundrum with all the different opinions and stories and products out there. If you use some sort of rust converter like the spray extend or Ospho phosphoric acid they maybe you can safely seal it back up. And those cases I would certainly use black Rust-Oleum first and then undercoat but regardless, existing rust or not, there are serious that any type of undercoating will eventually get cracks in it or chips and allow the moisture to get in and rust as much if not worse than leaving the frame alone. But what about paint? Doesn't it in theory do the same thing? But we don't leave metal exposed. We paint metal. Even then, it get small nicks and chips and the moisture does work into it and start to rust but it doesn't spread near as quickly as if you would have left the entire piece unpainted.
Then there's the theory of no painting or sealing at all and just coating with something like fluid film or some sort of petroleum product or a homebrew mixture. This doesn't leave the nice pretty appearance but preventing more rust is more important than appearance. You also have to do these on a more regular basis. It's quite a hard decision to make. I really want to keep this one vehicle rust free for the next seven or eight years because I really just can't go out and replace it at least not without searching all over the country and paying a lot of money for one.
