For what it's worth, I elected to use rattle can, Rustoleum automotive primer to cover completed, or semi-completed portions of the chassis as I went along. It's much easier to remove the primer with a wire wheel if you need to, rather than oxidation that has built up over time. That is especially true as the chassis becomes more complete (more joints, tubes, brackets and plates) with time.
I live relatively close to the ocean, so rust creeps in pretty quickly on bare metal.
I used 3M blue painter's tape to keep strategic areas as bare metal, but protected from rust, so I'd have a place to ground my welder. From sad experience, I've found only the more expensive 3M tape actually will come off pretty clean after a year or two.
The cheap painter's tape like from Home Depot is good for areas that you'll only tape up for a few weeks at a time.
Cheers,