Factory undercoating

Resto Rick's is one that alot of people use. Supposed to closely duplicate original(personally have not used it, so can't say)
http://www.restorick.com/proddetail.asp?prod=Undercoat

Another option would be 3M #08864, used with a Schutz gun. (have used this and works well too).
You can get the 3M at any autobody supply.