I recently did some cutting and welding in that same area of my early A and wanted to remove as much old undercoating and paint as I could to prep it. I found that a couple of cans of oven cleaner melted most of the old undercoating off.
I will keep this in mind. Great tip. I have been using a torch and strapper. Then following that up with a wire wheel. I have so much cleaning to do still. I wish i could have just blasted the car lol.