I've used Por-15 black and silver and Black Velvet from Techline on headers and manifolds. Each is not 100% reliable BUT I like the fact I can touch it up as needed VS a coating.

I sandblast them (no glass bead) - clean them really good (don't touch them with you hands) hit them with a propane torch to boil out any grease oil and clean them again.

Spray product with a cheap touchup gun and partially cure them with a infrared or propane heater, then finish on the car. Por 15 will smear if just left to air dry.

Tried it once with a salamader heater but I had a reaction ( I assume from the kerosene)