Folks, Fat Mat and B-Quiet are marketed and sold as sound deadening materials.  I have no experience with Fat Mat but I used three 50-foot rolls of B-Quiet in my Westy. It did work to reduce the tinny sound of any tap on the Vanagon sheet metal.  Doors now thump shut and no longer clang.  Things sound solid.  The Westy still has that roof opening that is noisy but the body is quieter.  Tintops would likely show an even greater benefit.  I already had the Westy interior out for rust investigation (found none) so it was not much more trouble to do the soundproofing and I'm glad I did it. 
The other products being discussed in recent threads, the Peel and Seal and the Quick Roof and any other brand names mentioned, while they may serve the same function and look identical, are in the roofing section of your home improvement stores.  Do not go to Home Depot looking for any sound deadening material.  You are not likely to find it.  Look in the roofing department for this stuff because that is how it is packaged and marketed and sold.  Last time I bought it, it cost me about 50% less per square foot than the honest-to-gosh B-Quiet that had to be shipped from Alberta, and it seems to work just as well and is likely just a few minutes away from you.  In the roofing department. 
Stephen