For any intact areas that can be reached, this is the only product I have found that is holding up on several rust repair projects I have done.
This Eastwood Rust Encapsulator
Platinum seems to do a great job and hold up well. I have used the rust reformer and the Ospho, neither seems to work for any period of time. This is the product I use when the other repairs I have done in the past with Ospho have failed.
I have samples that have been left outside on steel for over 3 years and still not rusting.
My understanding this does not "convert" rust like other products claim, but rather encapsulates it to starve it from oxygen, which is what feeds rust. Here is a link to the product.
Eastwood Rust Encapsulator Platinum
A word of caution is to only buy what you can use, even the smallest amount left in the rim of the can when you close it back up will literally weld the lid shut and you cannot get it off...it is that strong when it cures. Also any air space in the storage container and it will turn hard over time.
As others have said rust inside would also need to be treated, I don't think this would help with that, but will do a great job preventing rust progression on the surfaces where it is applied.
Trav's suggestion makes the most sense if you are in a position to do that. Rust always wins in my opinion, especially at advanced progression.