Paint on an exhaust manifold will be a colossal waste of time. If you don't want the rusty manifold to show, apply some stove polish occasionally. It's basically carbon black like the soot that accumulates on a kerosene lamp chimney, in a wax carrier that burns away and leaves the black powder imbedded into the porous surface of the cast iron. The center of the intake manifold will also refuse to hold paint because of the heat riser valve.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!