i did a lot of research while i'm doin mine frame off. i went with a product called rust shield, by a company called SEM. this stuff worked great, and you can get a catalyst that makes it more resistant to acids, fluids, and gas. i recommend it, this stuff was for real, my fingers were almost glued together when i was done.
i looked into por15, acid dips, and all. this stuff ran me $90/gallon with the catalyst, which is a lot cheaper than the others, and it worked great.
you can look toward the end of my thread here to see some of it: http://www.wranglerforum.com/f8/yj-build-16980.html
oh yeah, i brushed it on, and thats the way i'd suggest doing it, and i would do it no a vehicle that is still together. do the best you can in prep, but it will work no matter what