Factory undercoating removal

This is what I use. Blast it with heat then scrapes right off. Do small sections at a time, around 12 x 12". Do not heat areas that do not have undercoating or you will warp the metal. You dont want to melt the undercoating, just heat it enough that it scrapes off.....doesnt take much.
https://www.harborfreight.com/propane-torch-91033.html
I had my car on a rotiserie when I did it and it was outside. You will need some ventilation if working indoors.