Advice Needed

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.