How to Repair Cracks in a Cement Board Ceiling

Sand the cracks with medium-grit sandpaper to smooth the edges.

Apply latex-modified mortar, also known as thin set or use joint compound with hydraulic cement as an ingredient with a putty knife. Press the thin set or joint compound deep into the cracks. Extend the compound or thin set 1/4-inch past the crack on each side of the crack with a small trowel. Make the layer as thin as possible while covering the crack completely.

Allow the thin set or joint compound to dry fully, generally one to two hours.

Sand the repair surface with fine-grit sandpaper attached to a sanding block to level it with the existing ceiling.