If you want all of the rust removed from all of the blind channels, recesses and closed areas your best bet is having the parts dipped and chemically cleaned and rust treated. Not cheap but the ultimate solution to rust.
If that's not possible you can always clean off as much surface rust as possible and then use a rust treatment coating. Personally I prefer the Eastwood Rust Converter and Rust Encapsulator for such things because they are easy to use, easy to clean up tools and work well. I treated all of the rust in my truck with these products.
It's the rust you don't see that's going to come back to haunt you.
