This might be a problem. Oh well, keep grinding!

I use a razor blade also to remove old paint. If a car has been painted a half a dozen times, the razor blade will get it down to original primer pretty easily. If I find a wad of filler, I hit it with a propane torch or heat gun and get a gasket scraper up under it and it pops right off without enough heat to warp a panel. It doesn't take long to find out if a car was fixed back in the day by a skilled body man or a sculpture.....lol.