Some lack of understanding here. I'm no paint and body guy but I know you don't "neutralize" acid with water (as keeps getting brought up). You can dilute it or rinse it away with water. To neutralize it you'd have to counter it with an alkali like baking soda or soda ash. In practice, the reaction between it and rust sort of "neutralizes" its acidic properties. (OK, not really neutralizes it. Re-read your old high school chemistry books for what is really going on.) The residue left after treating with phosphoric acid is mostly ferrous phosphate which has a mild rust retardent effect. Phosphoric acid combines so readily with water that any residue is easily rinsed away. As for whatever other additives may also be in the mix, I have no idea what they might be but I suspect it's best they be washed off. Scrubbed off, if need be. Below 94 degrees and once the water has evaporated out of it, fairly pure phosphoric acid returns to its natural state. That of a whitish solid.
Stuff like Ospho containing mostly phosphoric acid is meant to be a rust remover/metal cleaner. As such it pretty darn good stuff and has long been used by steel mills for that purpose. The rust "converter" stuff I've seen (and tried) is pretty interesting but is pretty much a band-aid. It stops the rust, for a while, and does a pretty good job of covering it up. Also for a while. Sooner or later the rust alway seems to come back through the "converted" area. Or did when I tried it anyway.