Getting ordinary paint to stick on galvanized metal is a common problem that has never completely solved, as far as I know. The most common means of minimizing paint failure has been to apply on the clean galvanized surface a preliminary first coat of a weak acid, most commonly ordinary household vinegar. Whether or not the car builders routinely did that, i.e. cleaned the surface AND applied vinegar, I know not (but probably someone does).

Denny

Denny S. Anspach MD
Sacramento