Marine Clean is awesome stuff. You mix it with water to different strengths. It's not cheap, but it one of the best products I've ever used. We clean my buddies 72 Jaguar interior up with it, it looks almost new.
You lost me on what you are trying to do. Are you going to clean the bottom of your car with a propane torch? Then wash it with high dollar cleaner? I have less than $5.00 cleaning 50 years of crud from the bottom of my Falcon using soap and water. As you can see it the pic it cleaned the sheetmetal down to the factory primer.
I'm going to clean the bottom of my car, then use a wired wheel thing to scrap everything down to metal, then apply the rubberized spray. I was just asking the best way to remove the build up before removing the paint.
once you get it off, you may not want to put it back on. I would suggest cleaning all the undercoating off before wire brushing it. the undercoating will only melt and smear... I picked a cold day and mine just chipped off...
