If you put a rust converter coating, it will take care of it, but you will need to prime and paint after. Make sure to follow the prep directions. On my 72, I plan to do it in places that are not rusted through. Even if you think you got it with sanding cleaning, rust loves to return. Even trying to encapsulate it comes back to haunt us Bronco owners. After my frame is painted on the outside, I plan to spray a coating of the converter on the inside of everything to stop any further or future rust. There are several products out there but I like Barrier Bond. I have used it on a couple things and it worked really well. On a couple things, I just left it without painting it and still looks good.
David