I`ve heard Eastwood sells a good manifold paint that you brush on. The main thing is to clean every bit of rust and dirt off the manifolds before painting them.
Pah! I have used the Eastwood product and followed the instructions to the "T". The crap flaked off and I had rusty manifolds 2 months after application. I used POR 15 products as well and some Pep Boys paint as well. All with the same results. Grrr....
I finally had the machine shop that did my 340 coat my manifolds with Cast Iron Gray chemical coating. They also coated the inside of the manifolds with a thermal barrier to keep the heat in and moving out the pipes instead of warming up the engine bay. Cost was $150 for the pair, and they have been perfect for over three years now. They also coated my intake with a thermal barrier in the water passage, and a oil resistant thermal barrier on the underside of the intake to keep the heat off the air/fuel mixture.
The driver side is a pain because you have to lift up the engine a bit with the motor mount loosened, but it is worth it.