I had good luck letting the phosphoric acid to dry over night after applying it thick with a brush and stirring it around with the brush a couple of times in the first few minutes. I then came back the next day with more phosphoric acid which loosened everything up and wiped it off right away leaving a thin layer. This produced a nice fairly uniform gray layer that I applied epoxy primer to after it dried for a few days. No sanding needed. This worked very well for me. I agree that media blasting followed by epoxy is probably a better way to go if that works for you.
I couldn't deal with having to metal work and primer all the parts at once so I did them one panel at a time using the phosphoric acid to keep them in good shape until primered but more importantly, to get to any rust in areas I could not see or get some kind of tool into.
