KuRust? I've used it a fair bit. The idea is that it uses a chemical reaction to turn the rust back to metal, presumably turning Iron - Oxide, stripping the Oxide, to leave Iron. I've pretty sure it dissolves paint as well. So the idea is to stop rust on stuff you apply it, then when it turns black, paint over it.
The proper way to solve this is to get the car to a body shop and replace the metal.