The only problem with the zinc is it's affinity to lead, so you really need to be careful with the application process of the primer. Painting over galvanized steel should be undertaken with care and specific requirements. But you are looking for a finish color. I also consider the relative difficulty in applying IOZs. If they don't cure properly or if dry-sprayed, the subsequent coatings may delaminate.
Organic zinc is very applicator friendly.
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