I purchased this fluid film kit and added all of the extra wands and extensions.
http://www.kellsportproducts.com/Fluid-Film-Auto-Truck-Undercoating-kit-1-One-gallon-kit_p_13.html
Late fall before winter I put the truck up on the hoist and squirt this stuff over the entire undercarriage. Also put the flexible extension wands on and coat the living hell out of the inside of the rocker panels, cab corners, fenders, box sides, tailgate etc.
Im going on year number three and think I will need to get another gallon again finally.
It has quite a powerfull odor for the first week or two. But you eventually get used to it lol.
How well does it work? Well, let ya know in ten years or so. Thats usually what it takes for the fenders and rockers to disintegrate up here.