Man, you nailed it. I grabbed my old man's putty knife from his garage, and that thing was a beast. Took some elbow grease to get it clean, but the steel held a good enough edge. Had to file off a little burr on the corner though, or it would have gouged the mud. That saved my whole patch job too, no joke.
Yeah but old putty knives get dull fast, you really need a sharp one to keep from tearing the paper on the tape. A bent corner can also mess up the mud lines and leave you sanding forever. Did you hit it with a file or just use it as is?