Remove any chipped or flaking paint. If your concrete fireplace was painted previously, you may need to scrape down the old paint and scrub with a wire brush to remove old paint and create a smooth surface. If you've scraped off flaked paint, vacuum the area to remove any dust before painting.
Repair concrete cracks. Concrete filler comes in a bottle or tube and has a consistency similar to caulk. It can be easily squeezed into small to medium sized cracks. Fill any nicks and cracks with filler and allow to dry completely before proceeding.
Seal the concrete. You may not have moisture control issues in an above-ground concrete fireplace, but with any concrete there may be some seepage of moisture when the atmosphere is wet. Sealer will prevent paint from flaking off due to condensation from the concrete.
Apply concrete primer. Some masonry primers have a built-in sealer, so you may opt to purchase a two-in-one sealer-primer. Make sure any primer you choose is approved for fireplace use and adheres to local fire and building codes.
Paint the fireplace. Use a small paint brush to cut in around the edges and a roller to quickly apply an even coat to the face of the fireplace. Two thin coats generally adhere better than one thick coat.