On these, I start by using a wire brush to clean the exposed threads from the end of the nut. Soak this area with a good penetrating lube like K&W Knock'er-Loose.
https://www.oreillyauto.com/detail/c/kn ... w05/401724
The problem is once the nut is broken loose, the threaded shaft the nut is on will spin. If you look at the threaded shaft, it will have either flats on the end of it or will have a Allen hex in the end of the shaft. This is so you can hold the threaded shaft from spinning as you loosen or tighten the nut. With a good air gun, once the nut is loose, you may need to keep working the nut off and on the threads. Back and forth, stopping to apply more lube when the nut is tighten back in. You may be able to work the nut off the rusted threads but wire brushing around all the threads really helps.
If your air gun can not break the nut loose, you need to use only a socket, as like you said, all the hammering energy is used up in all the joints when adding more extensions and adapters. In this case, you can not do that. I then use a box end wrench and a dead blow hammer to try and break the nut loose. This works most of the time to get the nut loose.
If that does not work, I use a air powered zip gun and chisel bit to split the nut. This does two things. The vibration from the zip gun breaks the rust free.
Vibration is your best friend when getting rusted items apart. This also splits or opens up the nut, making the nut bigger, which is why the wrench does not fit the nut after doing this but the nut will be loose and come off. As with any tool, the more you pay for it, the better it is to use, which is why you see such a large price difference in air zip guns. The main thing is how good the gun is at regulating the speed as you pull the trigger. A cheap gun just is on/off and is hard to control.
https://garagespot.com/best-air-hammer/
If a air zip gun is not in the works, then you can try a nut cracker or nut splitter. This goes over the nut and is tightened down, splitting the nut open. They come in different sizes, to work in tight spots with different size nuts.
Here is one example:
https://www.oreillyauto.com/detail/c/ev ... ert0/65126
I forgot to say: Besure to lube the threads on the nut cracker and work the lube into the full thread area.
In rusty areas, it can be faster to just sacrifice the nut to get things apart. I will end by saying use anti-seize on all the new part threads. This will make the job a lot easier to take apart the next time. You will thank yourself later for using the anti-seize now.
