Unless you're working with a brand new vehicle fresh off the car hauler, the first thing to do is steam clean or pressure wash everywhere you intend to coat. otherwise, the undercoat simply traps dirt, road debris, and salt against the under body where it can do the most damage.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!