just last week i had an old set of headers sand blasted then aluminium sprayed (comes out like a rough grey surface) and they came up a treat. infact i will soon be selling them right here on datsun1200.com, as they are not headers for use with twin webers (or throttle bodies perhaps) eg garbage
i took mine to Brookvale Abrasives, on Ada Ave in Brookvale (for those of u who know Sydneys northern beaches)

as for permenantness of the paint, i do not know - if any1 here knows about alum spray plz say!