From what I understand, it takes a vacuum pump setup an order of magnitude higher than what you have used before.
https://en.wikipedia.org/wiki/Ultra-high_vacuum
Something I believe that ETF inherited from Hawkeye, and even after that, the inside of the neck is flashed with a very reactive material like barium, zirconium, titanium, and aluminum, and so on, anything to soak up what the pump could not pull out, or what is stuck to the elements.
As far as the gun itself, that's the hard part, replace the tired cathode, with the correct active surface on it, that also must be activated, the grids may be reused in most cases, put it all carefully back together hope that it all lines up, no cracks or leaks. Risky Business !