In my experience the best undercoating is what was used in the 60s and it seemed to be the same thing they used to patch cracks in tarred roads and it was also used on flat roofs with gravel. It was nasty as anything, smelled terrible and had to be applied hot. There are three things I've found today that aren't bad. One is a water based paint sold under the name XO Rust. It is black, kind of flexible when dry and thick. It is also real hard to find now and it is expensive even if you find it in a hardware store that happens to stock it.
https://www.amazon.com/True-Value-XO18-HP-Interior-Exterior/dp/B000C026CS That's what it looks like. I use that stuff in doors. Pour it in and turn the door one way and the other until all interior seams (where the skin and inner panel meet) are filled and the door is overall coated inside, too. Then let it dry really well and the door will be much less prone to rust even after a decade or so. Clean it as well as possible first and let it sit in the hot sun to dry for a couple of days before doing this. The other thing is a spray-on undercoating made by Permatex. It is the best of all the over the counter shake and spray products, but all of those have limitations. And the third thing is black plasti-dip. Thin it with naphtha about 1/3 naphtha to 2/3 plasti-dip and paint it on with an inexpensive chip brush. Wear old clothes because you'll get the stuff all over you. You can make it as thick as you want by adding coats, it is just like rubber once it dries and will deaden sound really well. But...it is very expensive. Good luck. Doing this today (just like in 1965) is hard work if you do it right. You have to start as clean and dry as possible and you can't just undercoat parts of things...coat everything you can get to. Water, salt, nothing will get through plasti-dip.
