Sometime around the mid-seventies Mobay Chemical Corporation developed Bridge primer that was one of the most powerful primers in the world. It was based on a simple principle that if you eliminated moisture and oxygen you cannot create rust and whatever rust was under it cannot continue. Resin produced at that time from Bayer chemical was compounded with aluminum flake. Rust is oxidation and the aluminum flake which is critical in this primer , Bridges coated with this system had a 20-year service life in the harshest climates. On my projects the last 30 years I prime everything with two coats of this silver primer then I go from there. In the late 70s it found its way into the automobile Hobby. It was sold as a rust preventative and a metal filler because it leveled pitting nicely. One day The powers-that-be decided that they are painting the bottom of their cars they would prefer black rather than silver and the black was born. The powers that be at that time didn't know what a carburetor was They also decided to change the resin, pigment and the solvent. When developed by Moby it was designed as a three coat system. Two coats of the silver primer and a color coati. . This photo is the Pittsburgh Homestead High Level Bridge 14 years less than 5% corrosion