greywuuf

There are ways.... Even if you have to sacrifice the knob. (file. Flats on it and use a wrench)

I seriously would try some automatic tranny fluid or some kerosene before you give up.
And consider a rubber mallet tap lightly on the knob as if it was a nail...careful the cast iron is brittle.....the object is a little vibration to loosen the rust in the threads.