A certain amount of air in cement mix is desirable. Air lets moisture expand and contract in a freezing environment without damaging the concrete. When you have bubble holes, also known as bug holes and honeycombs, on the surface of your casting, something has gone wrong. Your cement mix was too thick, the temperature too low, your mould release applied too soon, or you have poured the cement too quickly or vibrated the mould incorrectly. You can repair the surface cheaply and easily without recasting the surface or marring the finish.
Mix a new batch of cement or small amount of premix if the repair is small.
Clean debris from the surface and wet the repair area. Wetting the surface allows the repair batch to fill holes better and blend into the surrounding surface. Use an atomiser if it is a small or inside job.
Trowel and compact the cement into the bubble holes. If the hole is deep, wait 15 minutes or until the repair has begun to shrink, then resurface. Leave to dry overnight.
Water the repaired area lightly with a misting hose or atomiser.
When dry for the second time, buff lightly with fine sandpaper to minimise transitions.
Apply coloured epoxy with a putty knife for small holes and cement paste with matching pigment for larger areas when repairing an indoor concrete countertop.
Wet sand the epoxy/cement paste when dry.
Cement applied with a trowel will leave the surface smooth. For a rougher surface, brush wet cement with a broom, or sand with 80-grit sandpaper once the surface is dry. The repaired surface can be sealed or coated as you would normally once it is dry.