Undercoating
I tried using a wire brush but it didn't get me far so I just lightly grinded the worst of it off, splashed a couple coats of rust paint on it then undercoated it. I used color flo undercoating only cost me $30 for a gallon and did the whole truck and then some.
What I'm gonna do is use a wire wheel attachment to get all the rust off and when every bit o rust is off I'm gonna use a rust converter then prime it and finally use a undercoating spray lots of work but I don't know if I would even want a dealer to do mine
You can pick up a sand blaster pretty cheap at harbor freight. Don't underestimate the cost of consumeable items like sand paper, wire wheels thinner etc. Plus the value of your time.$400 may be worth it. Not to mention a warranty against rust.
I just undercoated my whole frame. I used a wire wheel to get the bigs stuff (the rust was pretty bad) then I sanded it all down till in was smooth and a coat of undercoat then a coat of spray paint. I just did a little bit at at time when i and time. I'll try to take some pics and get them up.

