Dust off the plaster with a rag to remove any loose particles.
Dip a sponge into a solution of warm water and several drops of liquid dish soap. Squeeze out the sponge so it isn't too wet and wipe down the plaster to more effectively remove dirt and debris. Let the plaster dry completely before continuing.
Apply a primer to the plaster with a paintbrush to cover it. Allow the primer to dry for several hours. This helps paint bond to the plaster to make the transition from wood framing to concrete more appealing.
Draw pencil lines on the dried primer to outline the design or details you want on the plaster.
Fill in the pencil lines with paint using a thin paintbrush. The pencil lines help prevent you from making mistakes during the detailing process. Acrylic paint gives you a durable painted finish that has a harder, shinier exterior.