If you are going to use any paint based system you really need to have the parts sand blasted if you want the paint to be durable.
I have used a stuff called Techline Black Satin (imported from demon tweeks). It is a small 120ml tin but I've done one 3S-GTE manifold, one turbo turbine housing, two downpipes and a set of Alfa 4 cylinder headers.
See attached pic. That downpipe was painted 10 years and 70000kms ago and the coating is still mostly OK. The O2 sensor nuts haven't fared so well given the ST205 bonnet grill means this area is exposed to the weather.
