my experiment down rusty lane has seen the following views

Chassi Saver/Por 15 - works fine on rusty parts, fair adhesion as long as the part is rusty. Flakes right off any non rusty metal after a year or less so I see it as a stop gap at best.

Phosphoric Primer (any brand will do I think) works good especially on not so bad parts and hard to reach places (spray bottle) but since I'm using SPI epoxy primer it has to be neutralized or it will come back and bite you. So I now only use it on unseen places, i.e. inside doors or the like and then I prime with rustoleum instead of epoxy

Just started using electrolysis and find its better than the phosphorus not as threatening to my eventual paint job and is easier to use... once the soaking tub was fabbed up which took all of about 20 min to make using scrap sheet metal left over from some panel replacement. I welded up a U shaped piece with the top nearly closed off and that's about it... Down side is nooks and cranny's are hard to get so.... back to a little phospor as touch up may be the final step for me in those instances.
In the end I guess we do what we gotta do to get that doo to be done.

Give me ambiguity
or give me something else