I haven't read throught the whole thread, so forgive me if I've missed something
If it's just surface rust, you can use
this stuff to convert it from rust to a paintable surface
It's
Rust Reformer from Tremclad and works great for simple jobs
I've used it @ work for years and it's as good as it says
If your rust is worse than surface stuff, be aggressive with the previously mentioned methods, and then use this stuff before you paint
It's paintable and, to a point, sandable