I know from using eucalyptus on other projects on the farm that the interior hard wood tends to be fairly rot resistant here on my farm, but the bark and inner bark quickly becomes soft and degraded. Thus I plan to remove the bark. I'm using a simple method of hammer & chisel to pop the bark off. It comes off fairly easily. I find that it's a rather boring job, so I'm only doing a few each day.
In the photo you can see that I've removed the bark. I'm saving the bark for future projects. Most will most likely end up creating soil in the garden, but some will be reserved for artistic creations of some sort.
I've had lots and lots of people suggest that I soak the rounds in some sort of wood preservative. After consulting a few local woodworkers, I'm finding the general consensus is to soak the rounds in hibor in order to slow down fungus rot and insect damage. I don't plan for these rounds to last forever, but I would like to have that patio around for 10-15 years. The patio will be built with good drainage, but it still will be getting rained on. So wet is something to keep in mind.
The rounds presently are sitting in the shade in an airy location. They surely won't be dried when we start setting them into place, but I don't want them getting moldy before we even start. Why not wait until they are really dry? That would take many months and since it won't make a significant difference, I'll be setting them into place while still green. I expect to see some checking, cracking, in the rounds but that won't make any difference either.
I don't plan to paint them or apply polyurethane. That would only serve to make them slippery when they get rained on.