When I cleaned up my engine bay a few years ago, I power washed the firewall and got the loose undercoating off. I bought the cheap asphaltic undercoating in rattle cans from my local parts house. It is basically the same stuff as was applied originally. If the old stuff broke out in large pieces, you may have to shoot a few heavy coats in the low spots to even it. It retains the original look for the truck, and even after a few years, still looks great. One thing that had me a little worried when I applied it was..... It goes on brown when you spray it on. But it does turn a nice matte black when it dries.