If you have a choice, I think Gunk Liquid Wrench or 3-IN-1 High Performance Penetrant over WD-40 (3-IN-1 is a WD40 brand, so you would think they have a reason to sell a dedicated penetrant). I'd say, leave it overnight.
I like that Alan mentioned "...that improtant first bit of movement." I think, by saying to turn the nut around, Alan was saying that the end of the spindle must not be hit directly with anything hard. Which is good (that is, imperative) advice. Consider the end grain of an oak block as the transition from your sledge hammer. After you're down to the last few threads that the nut can spin onto, remove the nut and drive the spindle directly with the wood block. If you get to where the end of the spindle is flushwith the plate, you might need to fashion an aluminum drift, else, you could try to grab the flanged end of the spindle and pull or pry. Hopefully, by this time, movement has become much easier.
Regarding, powder coating, there has been a great deal of discussion herein about whether or now the rubber in the bushes can take baking heat.
Finally, if I have said anything blatantly obvious, I apologize. It is purely and clearly form being a simpleton myself.
Richard L.