Whether you decide to stain or paint your planter box, it's time to apply the finish to this woodworking project. If you plan on staining your work, you'll need to apply polyurethane after finishing the stain.
However, if you choose to paint, you'll want to caulk and prime before applying the finish coats. Apply painter's caulk to all exposed gaps to help waterproof the unit. Once the caulk has set, you can apply primer to the entire unit, either with a brush or with a sprayer. Cover the entire project with at least two coats of a quality primer, to seal the entire unit and help waterproof the planter. Once the entire unit has been thoroughly primed, apply a few coats of your finish color until you have reached the color and finish you desire.