Fill a bucket with water and add some mild cleaner. Saturate the sponge with water and clean the brick thoroughly to clean the surface of any residual dirt and debris. Rinse with clean water and allow to dry completely.
Apply painter's tape to any areas you don't want painted. Apply one coat of masonry primer to the surface of the brick with a paintbrush. If the brick was previously painted, use a bonding primer. Let dry completely. Apply another coat of primer to the surface and allow to dry.
Apply one coat of high-quality latex paint to the surface of the brick with a large paintbrush. Allow to dry thoroughly. Apply one more coat and allow to dry. Remove the painter's tape. Let the paint on the brick cure for several days before using the fireplace.