We have built a large plywood box in which to put a large Umbrella Plant. We want to water seal the sides and bottom of the box (excluding the drain holes of course) and do not know what to use to do this. My husband said he thought there was a paint on sealer (like used in clay pots) but we cannot find where to get this type of product.
There are deck sealants and waterproofing products that may work for your project. Check for them at home centers. Keep in mind, though, that these products are basically water repellents. If you truly need waterproofing of the interior of the plywood box, you may need to line it with heavy plastic or EPDM (pond liner) to prevent constant moisture contact of the soil with the plywood.