1978 GMC Trucks Counterattack on Corrosion Sales Folder. How GMC fights corrosion. Cathodic Elpo Dip Process used on GMC Medium and Heavy Duty cabs and doors is shown. GMC's Extensive Anti-Corrosion Program. GMC is continually improving its anti-corrosion program. The result is a truly impressive anti-corrosion system for all '78 GMC Medium and Heavy Duty Trucks. The First Big Step - Cathodic Elpo Dip. Many Fasteners Get Anti-Corrosion Treatments as well. The list of items for various models is listed on pages 3 and 4. Condition: Wrinkling from exposure to moisture and some staining. See pics.