There is no easy, quick way. I heated about an 8 inch square area at a time with a handheld propane torch. Just enough heat to soften the undercoating, then scraped off the undercoating with a putty knife. Takes some time but did a good job of removing the old undercoating. Of course, I had the cab off the frame resting a the firewall on a cab dolly, made musch easier to get to.
