Worked at a restoration shop and several customers cars came in regularly for a through cleaning and detailing prior to show season, to include a manifold touch-up (we used the Eastwood stuff) It is OK but in a moist environment the manifold will still rust and you'll be redoing it every few years.
I think ceramic coated manifolds would have solved part of the problem. The one I pictured above still looks good after 5 years or so of being mostly inside but in the rain (no hood) at shows too.
Grigg