Cement

Mix batches of ferro-cement in recommended concentrations, such that you can plaster each batch on before it begins to set up. (Never add water to "re-constitute" partially-set mix.) Begin on the inside, at the top and work down, troweling on the plaster mix firmly-enough so that it squishes up between all of the nooks and crannies of the mesh. (However, be careful to avoid pushing lumps up beyond the outer layer of mesh on the outside. Trowel down anything that squeezes up.) Keep a "wet edge," preferably having one person mix while another trowels. This will add to the strength and integrity of the building.

When you are done with the inner level of plastering, allow it to cure before you begin the outer layer. The interior should be finished smooth. Depending on how it applies, it may be necessary to add a later, finish, coat. It may take some days to get a good start on curing, and the curing plaster should be kept damp and not allowed to freeze. This wait is not ideal in terms of bonding both layers, but it will be necessary for you to go onto the roof to do the outer layer and you don't want to be climbing on newly set plaster.

If you are concerned about the time between applying both layers, there is a bonding mix that can be purchased in home-improvement centers. Mix it according to instructions and apply before doing the outer layer. Remember, you want to leave enough depth of meshing to firmly secure the outer layer of ferro-cement plaster. Do the outer layer by starting at the bottom and working yourself around the dome in a spiral to the top. (By reversing the application sequence, as recommended, it is more likely that any water intrusion will tend to work itself back out toward the surface, rather than filtering to the interior.) Since all traces of wire and re-bar should be hidden, it may be necessary to put a finish-coat on the top.