How should I line my tin planter box?
Answer
Dear Maggie,
It seems that because you have used two different materials in building your box, you should consider addressing the different approaches needed for the different materials. Here are two ideas that I have found that you might consider adapting for both the metal portion of the box, and the wooden bottom. I think that once you protect the metal portion with a sealant, as described below, then covering the entire box with a pond liner would provide long protection for the box.
(From http://homeguides.sfgate.com/line-metal-planter-box-74689.html)A metal planter box makes a durable and stable container in which to plant vegetables, flowers, trees or shrubs. The metal does not absorb water and will not decompose. But the metal may rust when constantly exposed to moisture and it easily absorbs heat from the sun that may overheat plant roots. Lining the metal planter box with asphalt paint to seal it from water damage and installing a layer of insulating waterproof foam will solve these problems.
(From http://homeguides.sfgate.com/water-pond-planter-ideas-50794.html) A homemade wooden box made from cedar or other weather-resistant wood and lined with watertight material makes an attractive water garden planter. Painting the interior of the box with a rubberized sealant, being careful to effectively cover all surfaces, seams and nail holes, will make the box watertight. Another option is to build the box to the dimensions of a commercial water garden container or small pond liner and then place the liner or container inside the box.
Good luck with what will be a wonderful planter box!