Cement boards provide strong level surfaces for the installation of wall covering without the frailties of a wooden subsurface that cannot survive long in moisture-rich environments undamaged. Once installed, though, the boards require finishing for longevity and attractiveness. One finish you can use on cement boards is stucco. Extensive preparation is necessary, but once completed, you'll have a beautiful surface, suitable for further finishing by painting or left as-is for a raw plaster look.
Measure the surface of the cement board that you're covering with stucco, using a tape measure to measure both the height and width of the boards.
Cut sheets of fiberglass reinforcing mesh with tin snips to match the surface areas of the board. Add 2 inches to the heights of the mesh sheets for overlapping at horizontal seams and 6 inches to the width of the mesh for overlapping vertical seams.
Place strips of fiberglass mesh tape over the joints between the cement boards.
Cover the cement boards with a concrete bonding agent. Brush the agent in place with a paintbrush, using a moderate layer of the material. Press the mesh onto the boards, overlapping the mesh 2 inches where separate sheets meet horizontally and 6 inches vertically. Position the overlaps 6 inches from the edges of any corners when covering corners of the walls. Allow the bonding agent to dry, holding the mesh in place.
Apply stucco directly to the fiberglass mesh using a plastering trowel to spread the first scratch coat stucco layer in place. Coat the first stucco layer to a thickness of 1/4 to 1/2 inch. Allow the coating to harden slightly and go over the surface with a plasterer's comb to create a series of scratches over the stucco and reduce the layer slightly to about 1/8 inch in thickness. Allow the coating to harden for a further 48 hours. Keep it slightly moist while hardening by spraying the surface with a mist of water during the 48 hours.
Place a second layer of stucco over the first with the trowel, covering the scratch layer with a 1/8-to-1/4-inch layer of the stucco. Finish the stucco with any texture desired, brushing it with a stiff-bristled brush, for example, to create a brushed-coat texture. Allow the stucco to cure for 48 hours, misting the surface with water during the curing period.