I had to remove a bunch of undercoating when I worked on my truck. I did not find an easy way to remove the stuff. I used different techniques on different days. Sometimes I felt like sanding, sometimes I felt like gooping it up with paint stripper. Other days I felt like grinding with a wire wheel or a fiber 3-M stripper disk. Some days I used a stiff putty with a sharpened edge.
After many days of trying everything, one day I went out to my garage and the undercoater was all gone. Good Luck, it is tough stuff..
