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
We the willing, led by the unknowing, are doing the impossible for the ungrateful. We have done so much, with so little, for so long, we are now qualified to do anything, with nothing.