Window boxes add warmth to exteriors, whether on a country cottage or urban window. If you use a natural wood rather than a plastic box, take time, after arranging adequate support and drainage, to treat the surface properly. Cedar and redwood are the most weather-resistant woods. Non-arsenic treated exterior lumber will make a resistant box, but will be very heavy. Pine and exterior plywood are also common materials for flower boxes. All woods will last better when treated properly.
Sand all surfaces before beginning assembly, particularly edges and surfaces that will butt up against other surfaces. Smooth out the drainage holes with a rolled piece of sandpaper or rasp.
Apply an exterior grade wood sealant to all surfaces including the insides of the drainage holes. Exterior sealants can serve as a finish on woods like cedar and redwood, but ask for a paintable sealant if you plan to paint your box.
Allow the sealant to dry completely—up to two days or according to directions on the container—before assembling the box and finishing.
Wipe the box with a tack cloth or light sandpaper to smooth grain which may have risen during sealing.
Brush exterior-grade transparent or semi-transparent oil or alkyd stain on cedars and redwood; use oil or latex stains on softer woods. Brush on two thin coats rather than one thick one. Allow each coat to dry thoroughly.
Allow the stain to dry completely before adding a liner and plants.
Coat the box with enough primer to cover the grain of the wood. Use oil or alkyd primer for redwood, cedar and preserved wood. Use oil or latex for sapwoods like pine.
Smooth the primer with fine sandpaper after it dries thoroughly.
Apply one or two layers of oil base or latex exterior paint to finish the window box.