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Soak it real good for a day with some type of penatrant oil like PB or WD40 or Liguid wrench. Try to use a 6 piont socket or 6 point box end wrench to get a good bite the first time. Use a breaker bar so you can focus more energy on making sure the socket stays squarly on the nut. Depending on what fastener it is, sometimes you just pray that the thing just snaps off. Good luck. :smash:
Also, after applying and soaking with penetrant, if you have an air impact wrench, adjust the torque (or reduce the air pressure with a pressure regulator) low, and allow it to hammer (lightly at first) on the nut or bolt till it loosens and then you can back it off. The hammer effect can work very well. You may not want to hit it with full torque or pressure initially, as it may twist the bolt off rather than loosening it.
Oh good. I thought for a minute that this topic was about a different kind of nut that hasn't been screwed in 30 years. ;) :)
On to your question. I like to use plenty of penetrating oil, then get a socket on it, and apply pressure both ways. Loosen, then tighten. That tends to work the nut back and forth, breaking the rust loose, and allowing the penetrating oil to work in.