Cornices made of medium density fiberboard (MDF) are preprimed with a sealer that allows paint to flow and adhere to their surface easily. Older wood cornicework will need to be sanded and primed to receive new paint. Most paper-thin cracks will fill with paint and become unnoticeable. Fill larger cracks with a caulking or spackling compound and lightly sand away excess before painting. Glossy paints always look great on molding but can highlight mistakes; choose a flat paint to hide brush marks and rough spots or a semigloss for a hint of sheen. Use a good brush when painting cornices---a good quality, natural bristle sash cutter brush works well---remember the surface being painted is not drywall. One-coat paints are generally too thick for cornice painting. Thin them with water to gain the consistency of regular paint and use two coats, not one, for best results.
Paint With Brush
Paint a cornice trim that requires a separate color from the ceiling, before the ceiling is painted. Paint the cornice neatly with as little spillover onto the ceiling and wall as possible. If the cornice's color is a lot darker than the ceiling color, tape off the surrounding area before painting using painter's tape. Smooth, long, horizontal strokes work best for flatter cornice styles. Ornate cornice board will need a smaller brush to cover the crevices and edges well and should be done in sections. Cover and protect the newly painted cornice molding before painting the ceiling.
Cornices that will be the same color as the ceiling can be painted at the same time as the ceiling. If the crown molding is new and being installed before painting, spray or brush on the first coat in the garage or other area set up for spraying. Wipe down the molding with a damp cloth to remove dust and dirt. When spraying, use long, even sweeps at a uniform distance of around 3 feet from the surface. Spray in increments of approximately 3 feet and then release the spray nozzle, overlapping slightly when beginning to spray the next length of molding. Be sure to prime all unprimed molding first. Install the molding, fill nail holes with spackle and then apply the second coat (the first and only coat for the ceiling) once the spackle has dried. The first coat will see some damage during installation, but that damage is easily covered with the second coat of paint. Use an airless spray gun instead of a cheap hand-held sprayer for best results.