We discussed flattening earthbag walls in a previous blog post: Flattening Walls to Save Plaster
In that blog post (and the following Comments afterwards) I described some of the basics of this technique: tamp the sides of walls after several courses are complete; don’t wait too long or the soil will dry and be difficult to tamp; use enough clay in the mix so the earthbags are malleable; focus on eliminating awkward bulges; don’t tamp the walls completely flat – leave recesses between courses of bags so the plaster has something to grab onto.
The hyperadobe method uses mesh earthbags (see here https://naturalbuildingblog.com/hyperadobe-update-from-brazil/ and here https://naturalbuildingblog.com/mesh-tubing-for-hyperadobe/ — sorry, some blog posts have been lost). The plaster bonds perfectly to this mesh, so recesses between courses are not needed. This means you could flatten the walls on both sides to save additional plaster if you want.