You have to think about how short a razor blade is in relationship to the curvature of the bowl. The blade bends very little at any given time.
For really nasty messes......I hit the lid/bowl with heavy duty oven cleaner and let it sit for awhile (an hour in the sun inside a black plastic trashbag works wonders). Then give it a good scrape, and repeat. After 2-3 sessions, the worst of it should be gone. Then I move to dish soap and the steel wool. At that point, its easy to feel where the crud is. Anywhere you feel resistance, you need to scrub harder. Keep the razor blade handy for the few specs of stuff that is still stuck in place. After a little bit of time and elbow grease, the kettle should look almost brand new.
I have never scratched the porcelain on any of my cleanups, and I have been pretty agressive about cleaning some nasty shit out of the kettles