Why would you want to take it all off? isnt it there to protect the underbody?

 

To get it off, first I would scrape the majority off with a plastic scraper. Then I would try either mineral spirits, adhesive remover, or some other type of solvent to get the remaining residue off. That is the process I used to get 30 year old sound deadening material off the floor pan of my Camaro. It took a long time and the fumes get to you, but it ended up very good.

EVOlutionary