Don't do it. If you want to quiet down the car use dynamat. It's a sound deadener used in car audio. The cars that I've seen with undercoating have been nothing but a mess.
My dealer (who used to pimp undercoating) now says don't do it because (a) the factory anti-rust treatments are so much better now than in past years and (2) the after-market guys drill holes in your car to spray the gunk in, and the holes contribute to more rust. Must be something to what he tells me, because most of the after-market rustproofers in my area are out of business.