There are a number of ways to do this. Much of it depends on how deep your pockets are and how much work you want to put in. For the low buck route...Several gallons of aircraft paint stripper, a box of razor blades and a razor blade scraping knife works for large flat panels. The same paint stripper, a wire brush and Scotch Brite pads work on the irregular surfaces. Might need to add a heat gun or propane torch for the under coating removal.

For the smaller hard to reach areas, corners, etc, get a cheap suction feed blaster from Harbor Freight. Inside the frame areas, tape off the drain holes and pour a bit of Evapo-Rust inside to help take care of that. The rust transformer stuff also works well for those inside hidden areas.

Bad rust as always should be cut out and replaced. However don't fall into the trap of replacing entire panels because of one small bad area. Weld the patch in place and save what you can of the original metal.