I've done several decks and what I do is similar to what you're thinking.First,I power wash the underside,maybe wire brush the worst spots,let dry and brush on a rust converter.Let that dry for a day then do another coat and let dry for another day.Over that,I brush on a heavy coat of rusty red primer and recoat with another coat after a day.Done.Simplicity paint is costly and worthless under a deck.Save it for the topside using the same as above process.
Decks nearly always rust out from the underside from wet grass/dirt clinging to the suface dead air areas,usually close to the spindles.If you keep these areas clean and coated well the deck will never rust through.
