I've been using spray graphite on manifolds and headers for a few years now. Is that what that is? It works so great!! Looks like raw cast iron, easy to touch up. Spray on a rag and wipe. Or can be sprayed in the lid and put on with a brush. That's how I do the application originally if they are on the cars.
Yes, it requires occasional reapplication, but it's so easy. Prevents all rust, and looks good. Apply, wait a few minutes, then buff with a rag for nice shine.