Mario: While the price and large quantity might be tempting, I'd very strongly suggest that you use a material that's made for metal, in general, and for automotive use in particular. The problem with the stuff you want to use is that, for the most part it's very ACIDIC, which, when applied to bare metal has the unfortunate effect of just the opposite of that you wish to accomplish. Speaking from experience, I've seen cars that were 'undercoated' with asphalt sealer, literally rot away before anything could be done to save them. It looked like the thing was being attcked by termites! No joke. I'd think twice if I were you.

Brad