OK, it's phosphoric acid in a gel.
I've used it for a "quick fix", such as carport pilings, and I found by
accident that instead of washing it off after 15 minutes, when I leave
it on overnight or especially past a rainstorm, it develops a white
color, sort of like a slapdash primer job. It seems to inhibit rust this
way, too, although the label on the new bottles says it really doesn't.
I guess there's no reason to expect that; I'm not sure what the
phosporic acid and iron turn into chemically and what its properties are
vs. basic ongoing corrosion.
Now that I'm getting ready to actually get around to painting things
(with ACE Rust-Stop spray), is there any harm in painting over this
Naval Jelly layer? Or will it inhibit binding?