One thing that no one seems to remember is that hot dip galvanizing is just that...hot molten zinc. Thin metals, when dipped often warp and distort, and a true hot dip galvanized finish looks like one of those HD framing nails...burly, coarse and rough. I can't imagine that would be a good base for a car finish. Electro galvy (EG) is a smoother finish...but thinner zinc...maybe thats what car mfrs do.
There are also cold galvanizing coatings readily available. We haven't discussed them on this BBS before. Anyone have any experience with them on a car?
http://www.zrcworldwide.com/ there are others.
ZRC is an organic binder with suspended zinc particles, but it is mostly zinc particles. It is often used to touch up welds and field cuts on bridges and heavy commercial uses. I have seen it in local hardware stores.
Just another possibility.