I've used both rattle can and sprayed it with a gun. I think spraying with a gun lasts longer. As you stated, a primer coat always helps. I recently repainted the engine in my Firebird. The previous owner used the earlier, incorrect baby blue. I found a few rattle cans at the local Carquest and repainted what I could get to without pulling the engine. I am pleased with the end results. (And no, the exhaust ports don't last long). I recently saw in HPP where a guy painted the center exhaust ports with an exhaust manifold coating from Eastwood. They didn't get rusty or discolored, but turned a bare metal gray color. He said he would then touch them up every few weeks to keep them looking good. I believe Eastwood sells both rattle can and spray gun-type engine paints.
The rattle can style from Carquest is made by Plasti-Kote and is part #227 Pontiac Blue Metallic
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