Here’s a brief summary of the cement plastering process we used on the exterior of our earthbag roundhouse:
Step 1: Put cement between the bags in the gaps.
Step 2: Add thin coat of cement over bags.
Step 3: Add more cement.
Step 4: Add more cement.
Step 5: Trowel the plaster smooth and use a sponge float to smooth out the finish.
As you can see, the process is quite simple, although labor intensive. It boils down to adding a little plaster at a time. Leave each coat rough so the next coat will better adhere. No plaster mesh is needed — the plaster sticks to the earthbags, no problem. Resist the temptation to overwork the plaster. The total process took about five days of work for two workers spread out over a little more than week (some days there was no work). A third worker helped the last day.
Caution: Research the techniques used by plaster professionals in your area and follow their lead. The techniques in this video show what works in a rainy/humid climate, but they can cause problems in dry climates.
More videos at my Earthbag Video Channel.