Advice Needed

George Jets said
You have it right.

Sand and clean the channels, small drill bits with drill helps clean out the deep rust spots, small roll-lok discs to grind rust out, wire wheels on drills, sand with 80 grit.

Prime with good primer surfacer mopping it into the open seams. Let surfacer dry properly, then autobody seam sealer in the channel to seal it. Let it dry at least over night.

Then put your favorite automotive finish over the top.

Good as new.

3m makes a wire wheel for a 1/4 inch arbor that chucks in a drill. It is impregnated with some kind of resin that keeps the wires from folding over like they do without the resin, thus becoming ineffective. I searched the web and could not find them so I finally took some pictures of my own. They are really effective in your situation. Your best fix would be getting rid of all of the rust and use epoxy on the bare steel. Sandblasting being the final step before a direct to metal primer. Fusor 800 is an awesome seam sealer. It is direct to metal too.