Mike, Don't skimp on the initial coating of the bead blasted parts..You'll have a texture to the parts so not only are you preventing rust your dealing with the texture/rust pits through out the process also...Sometimes even a heavy coat dries to reveal the rough texture has come back to a certain degree...

The manifolds are a tough one technically you've already altered the appearance by simply media blasting them...And obviously they were bare/engine painted at the attachment points to the block..What most do is find the manifold coating/dressing of your choice {Eastwood.com comes to mind} and go from there..