eieio wrote:... hook a chisel blade into the outer circumference of the drain plug, and whack it in the counterclockwise direction
I'd not recommend this technique, as you only have topmost a quarter of inch of lever, hitting plug at the side.
By my experience best methods to release stuck drain plugs are:
- soaking it with some antiseize/liquid wrench spray few times
- hitting the plug with hammer few times to let rust crack a bit, but being careful not to damage the surroundings or housing. Impact driver might have similar effect, but honestly, it's a rough tool, very strong and I'd be careful with that.
- if still no avail, you can use some propane torch to heat it up moderately. Not too much, not till it glows red, but just enough to get some expansion and later contraction. Don't cool it with water or spray - just let it cool down, hit few times with hammer and try to undo again