[sick472]

I have used air chisels on a low pressure setting with a wide chisel to speed up the removal of undercoating-like goo. It works best in cold temps so that the goo is more brittle and less sticky and you just have to be careful not to get carried away and cut though the sheet metal. I do have an oscillating multi-tool that has a scrapper blade, but I have not tried it on jobs like this. It could work well, but cold and brittle may be the key as well. Otherwise, on a HOT summer day and a hand scraper!