[rdorman]

One is closed cell called Nitral (I beleive) and it is sold under the name of Star Gaurd. Other is any dense closed cell type foam that will hold up. Butyl and Nitral type products come to mind. Both are put on with a 3M trim type adhesive. The other choice is something like 3M rubberized undercoating and the truck bed liner type material. Be aware of overspray on these. 3M general purpose adhesive remover should take care of the over spray. Truck bed liner shops will do your wheel wells for you. There is also a brush on truck bed liner. I don't recall the name but I do recall it was a little on the pricey side. Cost? From $20 for the el'cheapo auto parts store undercoating to a couple of hundred for a shop to do it. I am going with the 3M rubberized undercoating based on posts that I found doing a search here at CC.

Rick