Prime with epoxy primer and use spray on adhesive to stick on the insulation. For insulation, I just used the foil backed stuff you can get at the hardware store. Of course, it's hidden by the headliner.
I wouldn't leave any metal bare in our climate. The epoxy primer seals out the condensation. Lesser primers can absorb water.
The 3M spray on adhesive is good. I stuck Dynamat insulation under the hood with it and it is not coming off.
