Cheap... get some POR IV - expose the rusty area/ bubbling paint. Lightly sand the area. The POR IV likes to stick to the rusty metal and seals it in. It is UV sensitive so you will need to top coat it with something. It will also flake off the good painted areas eventually if not covered. I got mine from a hot rod shop across form Broken Axles old locale in the NE- about $45/quart. I have also seen another product in an auto paint store recently but can not recall the name... rust something or other... I've tried the CT rust products too but the POR IV is much better. It will make a flimsy rusted piece of metal feel quite solid... but you will still have rust....
OR...
You can cut out the piece, slowly spot weld a new piece of metal in, and do some body work and paint, etc....
OR...
replace it with new part. Just don't do rattle can paint job if you go this route... Paint is too thin.