It's been my experience that phosphoric acid works fine unless the part gets wet. I treated my hood and left it outside (I work in my carport) to prime the next day. It rained that night and I was greeted with a completely rusted hood the next morning and an outline on the concrete where the rusty water has dripped off. Just surface rust but I still had to completely redo the hood.
Other parts treated the same way and stored indoors never showed any signs of rusting.
