Aluminum exhaust manifold?...........
Ok I have painted alot of water jacketed headers...none of which were aluminum....but if they were ...I would bead blast them them, epoxy prime them, and then I always use base /clear or single stage acrylic urethane...or polyurethane.....if they are water jacketed and have a good amount of flow, the only place they will discolor is on the flange itself..
As for paint over powdercoat....as long as it's scuffed well, it will work just fine...I do it all the time..
Rattle can paint is only for tagging bridges..:supp:
I guess VHT has some high heat stuff,also you can look around at Eastwood...but I dont think they would do very well.
http://www.eastwoodco.com/index.jsp