If you want a black undercoating check out Daubert Nox-Rust 121B. I believe it's the same product GM is using to coat the frames on some of their heavy duty trucks. It's well priced. It can be applied over existing coatings and rust.
NOX-RUST X-121 BLACK
NOX-RUST X-121 BLACK thixotropic wax rustproofing compound is designed to provide a long-term protective film for automotive primed and unprimed surfaces. NOX-RUST X-121 BLACK will dry to a black, firm wax-like film that will repel water, self-heal, and remain flexible at low temperatures. It is...
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