Turning a wood box into a planter is a fun project, but if you don't take a few precautions, the box will get wet, and the wood will start to rot. Want to have your homemade planter last as long as possible? Then check out these ideas: 1. Paint the box. Paint helps to seal wood from moisture and sunlight, and it can help your box last longer. Sand the box before painting it so that the paint adheres readily to the box, and if possible, choose paint designed for outdoor use.