I was thinking of using the stuff that NAPA sells on the business side of my inner fenders when I put the new ones on. I was thinking maybe it wouldn't rust quite as fast and keep the noise down from the stones that my STT's like to throw.

 

back in the midwest they had a spray on undercoating called ziebart that sure quieted things down. they could drill holes in the door pnls and inner fender pnls too and spray in there and put in rubber plugs. i think the stuff i had sprayed on my bed would sure keep it from rusting but probably no sound insulation. tc

 

I had bought an older Suburban that I found was from the east coast.

I had noticed there were a bunch of little rubber plugs on it.

Anyhow being here in Southern California where it never gets cold after a while I start to notice black tar oozing down the windshield and pillars.

It had the Ziebart treatment and it had started to get soft from the heat of the sun.

http://www.ziebart.com/rust_protection.php

Every cavity in the body was filled with the stuff including the underbody and drive shaft.

I gutted out the truck and spent hours with the steam cleaner melting the stuff out of the body and then gallons of gasoline wiping it down afterwards.

If applied properly I can see how it would protect your truck, but done excessively it can be a nightmare.
Jim