Wadcutter wrote:
Put down the long handle breaker bar.
Several days soaking with PB Blaster. WD40 isn't penetrating oil. After a few days soaking fill the water heater and turn it on so the water gets hot. It will heat to about 115*. After a couple of hours of the water heating then spray the nut with compressed air like for computer parts. The air will be cold. The heat of the threads and the cold nut will usually break free.
Tank will get to at least 130*F during heat cycle (T-stat pre-set for 130*F) which should only take 30 minutes or so.
Might not work due to plug with anode on it is the threaded part which should heat up and then when you spray with cold air the bung area that plug is screwed into will probably cool off first and contract.
Spray it with PB Blaster or Liquid Wrench for a few days and a light occasional tap on end of plug should loosen the rust.