Once the paint or polyurethane has had plenty of time to dry completely (at least 24 hours after the final coat), we'll turn our attention to lining the inside of the planter. We'll be attaching some 6-mil plastic and inserting a drain in this step of these free woodworking plans.
For this large planter, we'll need one single piece of 6-mil plastic (either clear or black is fine), measuring 6' x 10'. We'll attach the plastic by stapling it to the very tops of the plywood sides (just under the top trim). The easiest way to complete this task is to carefully center the plastic over the top of the box. Then tack one edge of the plastic all along one side with staples. Push the plastic down into the box, laying enough plastic to cover the side, bottom and opposite side. Then, staple the plastic into place on the opposite side.
The ends are a bit more tricky. We found it easiest to tack the end into place, then fold over the plastic that bunches at the corner against the end much like one would fold square corners when making a bed. Tack the flattened fold against the top of the box, then fold the opposite corner, again against the end of the box. Repeat with the opposite side. Trim off any excess plastic with a sharp utility knife.
TIP: Take care that you don't puncture the plastic when installing it, or else water may seep through the plastic and possibly damage the planter box. If you puncture the plastic, discard it and install a new piece.