Holy Rust Batman! What to do?

My utility trailer had the same cancer under it. I wire brushed the loose stuff, pressure washed it, let it dry, and then brushed on KrudKutter's rust converter. It worked for the most part, but needed 2 coats as it went on like water as it's a really thin product. Once it dried I washed the trailer again to get rid of any excess that hadn't adhered/converted rust, then went through 8 cans of Rustoleum gloss black spraypaint.
Been a year now and still looks good, but there are some rust pocks coming back - trailer lives outside uncovered, so this was somewhat expected. I've heard good things about CorrosionX, so I plan to treat the belly with it come spring time. Good luck on your tedious project

http://corrosionx.com/corrosionx.html