Cover the surfaces below the crown molding with plastic drop cloths.
Clean the crown molding with a sponge dipped in phosphate-free cleaner and warm water. Use a soft-bristle brush to clean crevices. Rinse the sponge in clean, warm water to remove the cleaner. Dry the crown molding thoroughly with a clean, lint-free towel.
Apply painter's tape to the edges of the walls and ceilings that are adjacent to the crown molding. This will enable you to paint the crown molding without getting stray paint dabs on the wall or ceiling.
Fill any gaps in the joints of the crown molding by scooping a bit of spackle compound onto a putty knife and carefully pressing it into each gap. Smooth the spackle with a damp paper towel, removing any excess. Allow the spackle compound to dry and harden overnight.
Sand previously painted crown moldings with 120-grit sandpaper, lightly sanding to create a rough finish. Also, gently sand any rough edges of the hardened spackle compound that you used to fill joint gaps. Wipe sanded surfaces with a damp cloth.
Stir the primer with a wooden stir stick. Apply the paint primer to the crown molding with a small, angled paintbrush. Avoid saturating your paintbrush by loading only 1/3 to 1/2 of the brush bristles. Tap the paintbrush on the side of the paint can, removing excess primer. For best results, use long, sweeping brush strokes, painting a few feet of the crown molding at a time. Allow the primer to dry overnight.
Stir the paint with a wooden stir stick. Paint the crown molding with an evenly-applied coat of interior latex paint of your color and gloss choice, using a small, angled paintbrush. Avoid saturating your paintbrush by loading only 1/3 to 1/2 of the brush bristles. Tap the paintbrush on the side of the paint can, removing excess paint. For best results, use long, sweeping brush strokes, painting a few feet of the crown molding at a time. Allow the paint to dry overnight.
Apply a second coat of paint to the crown molding and allow the paint to dry overnight.
Remove the painter's tape after the paint is completely dry.