No easy way out. Any chemical treatments will be a band-aid repair. Chemicals will only work on very minor rust where there is no pitting.
Think of the rust as a cancer on the vehicle. The only way to really get rid of it is to completely cut it out and weld in new metal. Replace whatever body panels you can with new ones if your budget allows and if they are availible.
If the body part is made of heavy material you could remove it and have it sand blasted.
If you can remove the part and it is small here is a proceedure I saw on the Trucks show on Spike TV.
Procedure to Remove Rust from Steel Parts Electrically
As Seen from Spike TV Trucks, Episode # TK2007-01,
Original Air Date: February 04, 2007
Items needed
•Plastic bin
•Sodium Carbonate – Washing Soda Used as a soap enhancer for washing clothes. The demo showed Arm and Hammer Washing Soda
•1 teaspoon per pint of water
•Steel rod or steel plate, plate would work better
•Battery charger capable of two amps
Instructions:
Fill the bin with enough sodium carbonate solution to cover part
Connect the negative lead of the battery charger to the rusty part. OK if part of the lead is in the solution.
Place the steel plate in the solution and connect the positive lead to the plate, outside the solution
Set the battery charger to two amps and plug in.
Bubbles will form on the negative lead and part. This is flammable hydrogen gas. Do this in a well-ventilated area and no smoking.
For larger rusty parts it may take two days, smaller parts will take less.
A black film will form on the part and the rust will be gone.
Clean off the film and paint.
The only products available to you will prevent rust, not get rid of it. The only option is to cut the effected area away and weld a new piece onto it.