Wood makes a lovely material for planter boxes. It's rustic looking, and the sharp smell of cedar can contrast beautifully with the smell of fresh flowers. Keep your planter boxes in tip-top condition by waterproofing both the inside and the outside.
Before you begin, collect these materials:
To protect the inside of your planter box, use your putty knife and apply a thick coat of roofing cement to the inside of the box. Apply the cement along the entire surface of the inside of your planter box. Allow the cement to dry, per the manufacturer's instructions. After the cement has dried, use your drill to create several drain holes in the bottom of the planter. Most flowering plants do not like sitting in stagnant, standing water. Creating drain holes will help you keep your plants healthy. Next, cover the drain holes with landscape paper, to ensure that the soil doesn't run out of the planter.
Protect the exterior of your planter box with a water-based acrylic sealant. These sealants don't produce a significant amount of harmful gases as they dry, and spills clean up with soap and water. These sealants also dry relatively quickly, meaning you may be able to apply multiple coats in a single day. Apply an even, thin coat of sealant to the exterior of your planter box, and place in a sunny location to dry. After the first coat has dried (consult the instructions on the sealant can), apply a second coat of sealant and allow the box to dry completely. For a more colorful look, apply two coats of desk paint to your flower boxes instead of sealant. Desk paints come in a variety of sources, and are well suited to protect the wood of your flower boxes.