i spray the inside of all my chrome bumpers, including the side pieces and the insides of the over-riders with underbody spray. i sand em down if they're rusty just enough to clean up a bit. any overspray can be wiped off with mineral spirits.

then i use anti-seize on the bolt threads. i'm tempted to spray the bolts too. i'm dealing with a bunch of rusty bumpers at the moment. they just don't hold up very well, and rarely am i able to reuse the bolts.