Painting headers?
Did you use the Por-15 primer?
I did some manifolds last year, but haven't started the car yet.
I sandblasted my manifolds as best I could.
On one, I used Rustoleum rusty metal primer, on the other- just the Por-15 "Metal Mask".
The color of that metal mask is dead on for cast iron. It's so close, it can be difficult to tell what's not been painted yet. I obviously left some areas with too light a coat, and there is a small ammount of rust in a couple of places, on the one I didn't primer.
How long ago did you do this, and how are they holding up?
I did a cast iron water neck on another car over a year ago.
I've driven it a couple of tmes, and it still looks great. No primer there, either.