I have used the Eastwood Chassis Black Satin on the whole underneath my car. I also use to spray all the components on my chassis and other parts like brackets.
About etching primer............my understanding is that is to be used over bare metal only. That way it can "etch" the metal for a topcoat.
I have found that if you "etch" any bare metal with a good sanding and apply a coat of good primer you are good to go.