Add style and prolong the life of your window box, especially where exposed to high sunlight levels, by painting, staining or applying an exterior oil or urethane. Follow the manufacturer’s instructions for the coating that you apply it to all exterior surfaces. Make sure to coat the bottom and about one inch of the interior from the top edge. Staining changes the appearance of the wood and can enhance the natural wood grain. Consider adding a few coats of urethane on top of the stain for a glossy and warmer finish that repels moisture but realize that a urethane finish will need to be reapplied once every 2-3 years to maintain its shine if exposed to intense sunlight.