I've read a couple of posts about whether and how to remove the factory undercoating. I had some "bald spots" on my undercoating, a bunch of dirt packed in the texture that wouldn't come out even with a scrub brush, and a fair bit of overspray from the recent paint job that all added up to things looking kind of crappy, so I thought I'd give a go at removing the old stuff in the rear wheel wells. I have a trigger-ignited propane torch that I've used for soldering copper pipe that was handy, so I put that to work along with a relatively thin 1" wide scraper. With the torch in one hand and the scraper in the other, I found that keep the flame just ahead of the scraper was the best combination to soften things up without putting on too much heat. I moved from one end of the wheel well to the other every couple of minutes to avoid overheating the metal, and turned off the flame about every 5 minutes to let everything cool off (especially the scraper). The was a little bit of smoke, and the globs tends to a glow just a bit before they came off, but nothing burned. If you're reading this and thinking about doing the removal this way, I would offer 4 pieces of advice:
0. Have a fire extinguisher handy, just in case.
1. Be very careful not to push too hard with the scraper, as there is a risk of your hand passing into the flame if you hit a really soft patch.
2. Wear long sleeves. A couple of hot globs landed on my coveralls without a problem. One glob landed on bare skin and left a hickey (I'm pretty sure I won't have to explain this one to my wife).
3. Keep the flame directed in a safe location or turn it off if you're checking progress. It would be pretty easy to accidentally wave the flame somewhere you didn't want it to point... like at your new paint (no, that didn't happen).
The scraper started out flat but ended up developing a curve as I used it. It actually worked a little better with the curve in scooping out the material.
This approach removed most, but not all, of the old undercoating. The picture below is a "bottom up" view of the wheel well. I'm using a truck bed liner material applied by brush and roller to recoat the wheel wells. I'll add some pictures later when finished. Some of the edge lines from the scraper are telegraphing through the new material, but I think another coat will hide most of them.
An argument can definitely be made to have left the undercoating alone, but I really didn't like how it looked. The front wheel wells are in much better shape and cleaner, so I'm going to just scuff that stuff a bit and recoat them.