Here is the pertinent information from Dan's link, above:
Spray Manifolds Gray
Exhaust manifolds may not be the ultimate in flow, but they fit right, they usually seal well, and they look stock.
Unfortunately, they also usually look like hell, thanks to rust. You've probably tried to paint them only to have it burn off and look worse. Try this: Pull the manifolds out, strip them down to clean iron, either in a sandblast cabinet or with a wire brush, then get a can of spray graphite lubricant and follow up with a soft rag to rub it in. The spray is powdered lead in an oily binder, and it will make the manifolds look like fresh, bare, cast iron, but it won't burn off. When it eventually gets to looking shoddy again, get out the spray can and touch it up.Sounds like good old high-quality wood-burning stove blackening; or,
Calyx Manifold CoatingDoes anyone have first-hand experience with this type of product?
