If it is the asphault based crap, you can hand sand it or maybe Eastwood has some sort of nasty chem you can use to remove it. The original thick undercoating in the wheel wells is like this. I hit it with a die grinder and wire wheel, and it just smears around when it heats up.
You really, really, really don't want to get me started on this subject! That dang 'rustproofing' does exactly the opposite of what it is supposed to do. My Alpha Cat had the undercoating, and all it did was trap the water between it and the metal. Rusted out real fast! The same thing with the 'gravel-guard' stuff they put on the lower surfaces of my 85 Turbo Coupe. By the time you noticed that it was swollen with rust underneath, it was too late! Oh, and the underside of the Turbo Coupe was not rustproofed and it was rust free when I gave the car away...
Anyway, back to the rustproofing removal, I was able to scrape a lot of the rustproofing off with just the putty knife without resorting to heat. A lot of it was hard and brittle and removed in big chunks. I did the heat and scrape with what was left of Alpha Cat -- that is what was left of the car after I replaced the floors, front inner fenders, rear inner fenders, etc. I used a bernz-o-matic torch with the flame spreader attachment and a putty knife. I cleaned up the residue with kerosene followed by a lacquer thinner wipedown. Actually, I got disgusted in the middle of all this and that's how the car sits seventeen years later. :realmad:
Of course, you have to remove all of the interior of the car, including the carpet and sound deadener and also keep an eye on wiring harnesses! You also gotta be careful around the fuel ines and brake lines on the underside as well. You really don't want your Cat going up in flames!
I rented a big steam cleaner (hot) from home depot.I cleaned every thing.It cost me 50.00 for the day.Take note-do it where you have alot of room(blank)flies all over the place,It took 20 years of dirt and under coating off and It cleaned places that were hard to reach. Dan