Sand the painted beams lightly with a fine-grit sanding pad to remove the sheen. This enables the paint to adhere to the surface. Wipe off any dust left from sanding with a tack cloth.
Apply an even coat of the light color of interior paint to the beams using a paintbrush. Allow the paint to dry for at least 24 hours.
Brush on a coat of the dark color paint over each of the beams with a paintbrush. Wipe the surface of the beams from one end to the other with a cheesecloth to streak the paint.
Rock a wood grain tool across the surface of each beam from one end to the other to make the wood grain onto the beams. Do this slowly and with purpose.
Allow to dry for 24 hours.