Since the early 1930's, most north American cars have been treated with zinc phosphate prior to painting to enhance paint adhesion and rust resistance. The zinc phosphate will tend to heal minor scratches and chips that go to the metal surface, it can't mend the paint but it will slow the tendency for rust to form. In the late 80's manufacturers started using galvanized panels in rust prone areas of the vehicles. Paint adhesion was a problem and chemical companies struggled to modify the zinc phosphating systems. Between the early 70's and early 90's the salt spray resistance more than tripled. PPG's cathodic electrodeposition primer also enhanced salt spray resistance and allowed paint to be electrically charged into areas that could previously not be reached with primer. Prior to the 70's, the only galvanized parts were brackets and rocker panels. Some southern assembly plants used iron phosphate because it was a cheaper process to enhance paint adhesion but did little to stop rust. Vehicles made in these plants were not supposed to be sold in rust belt areas.
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