I'm considering how to prepare a barrel for a phosphate immersion, not the naval jelly trick. Obviously, you don't want the acid etching into the bore, so you must plug it. I have read that FAL builders who park their own have used things like PVC tubes (???) to save on expensive park tanks, so obviously they are doing something.
How does one plug the barrel? Is it plugged on both ends? The only way I can think that people do this, is to plug it one end at a time... plug one end and the gas port with a rubber cork, and partially immerse the barrel, then repeat the process for the other end. If the immersion line is on the gas block's journal, then there will be no "rings" or inconsistent coloring.
Is there a way to do this in one pass, with both ends plugged?