I researched something back when I was a kid. I grew up in Syracuse, New York. This area on the planet is plagued with corrosive conditions. Most vehicles rust out within a few years, leaving just the engine in good shape. To counter the effects of such a corrosive environment I researched the ingrediants which rust proofers used. I worked up a mixture of Rustoleum and Linseed oil to seal the interior and exterior of all my vehicles. You can spray or paint this sollution on. Plus you can choose any color you like which adds a nice touch to the undercarriage. Most vehicles I have applied this to only cost me about $25 to $30 dollars. Nice thing about this sollution is you can make it as rigid or flexxible as you like. This sollution is water tight, corrosion resistant and cheap. I always remove my carpets and apply this simple sollution. You can apply your carpet on a semi tacky sollution to get perfect seal. My undercarriage now is desert tan. I will probably change it to a darker brown soon. Use 60% Rustoleum w/ 40% Linseed oil for semi rigid (Common). Use more linseed oil for more flexxibility (hoses/brake lines). Use more Rustoleum for rigidity (Frame/bumpers).