ok, here we go... on the cheap...
your biggest issue is the stopping the rust... we'll start there.... i would mechanically remove as much as possible, DA with 50, 60 grit.. grinder, flap wheels, wire wheels, etc.... if any is left, I'd probably throw some converter on it before a primer... a qrt of ospho would do the roof easily ... re-DA with some 150 to 180 to put an etch to the treated areas..... prime.. epoxy is a good choice for raw steel..
just keep in mind the old adage, "your paint job is only as good as the substrate"... so a perfectly sanded smooth primer will yield the best paint work... just make sure everything is at least scuffed, no shine...
as for the rest... yeah, powerwash it and blow off as much as possible.. than take a DA to it.. something around 180 is good.. see how it feathers at the peel areas, it may continue to fall off.. sand till it's adhered and actually feathering.... these feather areas between layers are actually the most important part to get some primer on if you can.... and keep to a minimum... those open edges are what will show thru, or worse, solvent cringe....
prime the rest as you see fit.... DA or wetsand out everything with 320 or 400 for paint... the longer you can wait between sandings/coats in your topcoat, the better... I assume your running some sort of air-dried enamel like a rusto or tractor paint.... stuff takes forever to dry, and doesn't sand particularly well, so patience is important... do not try this with a metallic..
any ??? ya think I can help with, holla...
