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The main purpose of a primer is to prevent corrosion. The most corrosion-resistant primer is the MCU aluminum pigmented permanent rust sealer/ surfacer. When doing bare metal metal prep is used first then followed by two coats sealer/ surfacer. followed by urethane or epoxy surfacing primer on body panels. On brackets, frame components two coats of silver one coat of color. Here is a comparison according to the DP 40 product data sheet two coats of DP40 , 5 coats of color and two coats of clear will pass a 500 hours of 5% salt Fog testing. The Three Coat Polyurethane system ,14000 hours of salt fog testing. With A EPOXY you need 65 degrees or above to use it. MCU NON LEAFING from 34 degrees up to 90. This is the most anti-corrosive primer used in the industry