Please take the time to try a needle scaler. If you have to work with rust / dried mud / hard undercoating this tool will be your friend. Solvents dissolve undercoating but leave a mess. Scrapers and chisels can chew up a piece of metal. The needle scaler will vibrate rust or tar right off a surface.
I use mine all the time so I have an Ingersoll Rand version. But there are some pretty inexpensive ones on the market:
https://www.harborfreight.com/compac...ler-96997.html