I keep a 50/50 mix of automatic trans fluid & acetone for breaking stuck parts free. I dispense it from a condiment bottle I bought at the dollar store. It works as well as PB Blaster or Liquid Wrench, but costs much less in the long run...and you'll probably need a lot. Allow it to soak into the joint and perform its magic--reapply often. With luck, things will break free with no damage or scarring.
Some folks like to apply heat from a propane torch to the outer tube, but I prefer to avoid that if I can.