The very crown molding that adds character and substance to a room also makes painting the room a little more difficult. Uneven lines on the wall or paint marks on the molding can ruin the luxurious look that the trim should achieve. A nice clean, sharp line, however, can enhance the look of the room and highlight the molding.
Sand any rough spots along the bottom edge of the molding, and fill any holes, dents or nicks with wood putty. Let the putty dry and sand lightly until it is smooth. Clean the wall and the molding with a damp rag to remove dirt and grime so the paint can stick. Be sure to remove any dust along the bottom edge and underside of the molding so that the painter's tape can also adhere. Let the wall and molding dry completely before taping and painting.
Apply tape to the bottom underside of the molding so that the edge is flush and even with the wall. Let the outer edge stick out past the molding while applying the tape. Press the tape firmly against the molding with a putty knife, making sure there are no loose edges near the wall. Fold the outer edge up against the molding and smooth it with your fingers.
If you recently painted the molding, or have paint available that was used on it, apply a thin line of this paint along the edge of the tape where it meets the wall using a small artist's brush. The matching paint will seep under any loose points in the tape, where it will be undetectable due to the matching color. It will also seal the edge so the wall color cannot bleed under the tape creating a neat, sharp line. Let the bead of paint dry and then paint the wall as usual. If you don't have the paint used on the molding, cut in or paint the edges around the molding with a high-quality paint brush. Choose a trim or sash brush with natural bristles to get the cleanest line. As you're painting, make sure the tape is still secure and there are no bumps or bubbles that would allow paint to seep under the seal. Paint the rest of the wall as usual, using the brush to cut in the edges and a roller to cover the rest of the wall.
If you have a light touch and steady hand, you can use a paint edger to get a nice clean line along the bottom of the crown molding without using tape. Dip the edger in paint, scrape it along the edge of the container to remove excess paint. The pad on the edger should be loaded with paint, but not dripping wet. Paint with the edger along the top of the wall, just underneath the molding, applying only enough pressure to lay on the paint, but not so much that it drips out the bottom of the edger.