I've used the Calyx product with extraordinary results. Bought mine from Eastwood but other suppliers have it and it's a little cheaper.
I've used it on intake manifolds, exhaust manifolds, steering boxes, and master cylinders. I removed and media blasted some. Others I just wire brushed a little and applied the product with a toothbrush leaving the part in place on the vehicle. At $20 for a small jar I thought it was a bit pricey but after everything I've done I still have over 2/3 of the jar left. Just make sure you work the product into all the little crevices and continue to brush to a nice mellow shine.
I previously had exhaust manifolds powder coated but it was pricey and they didn't come out looking nearly as nice and natural. Plus you can touch up your work later if a problem arises (hasn't happened yet).
