Hi Erica, thanks for the update request. I have been using the system again this spring and have been wanting to share how it's doing, so I'm glad you bumped this up.
I have found that I don't need any protection beyond the flat stones over the top. If I had detailed the bench top a bit more during the installation of the top stones, and paid particular attention to overhangs, I feel like it would last indefinitely. At least, in a way that I find acceptable. I understand that in this day of perfect modern finishes this would fall far short of acceptable for a lot of folks.
Anyway, I think it's important to remember that a system like this differs from traditional
cob structures and what is necessary to protect them in that these have internal power drying systems built in. Also, the half barrels as internal bench support/molds means that the cob is pretty darn thin in section almost everywhere, so it doesn't have the capacity to hold
water in deeply like a large cob mass would. I believe those two factors go a long way towards mitigating continuing decline, mold, and mushy bits.
I made a little
video, rather than typing a long treatise, although it seems I've done a bit of that anyway! Hope this answers some questions.