If you followed our recommendation when building your library, you used ¾” double-sided MDO (Medium Density Overlay) rather than regular plywood and should have enough left over for a mounting board. MDO is a high-end specialty plywood with a heat-bonded resin on both sides specifically designed for long-term exterior use. Developed for signs, paint finishes on MDO are up to three times more durable than the same finish applied to regular plywood. Plus it’s an absolute dream to work with and so worth the added cost. As an added bonus, MDO is 100% waterproof provided the edges are properly sealed. 

 

Cut a piece of MDO two inches smaller (in each direction) than the bottom of your library. Fill all voids around edges with wood putty, allow to dry, then sand smooth. Lightly sand around all edges and slightly round (or "smooth") all sharp edges. Using a small brush or sponge, generously paint all edges with a mixture of half water-proof exterior glue and half water. Wipe away any drips as you go. Allow the mixture to soak into the edges and dry. Repeat, then lightly sand. As long as you are using MDO, it is not necessary to paint the mounting board.

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If you are using solid wood or regular plywood (at least 3/4" thick) be sure to thoroughly seal the wood by first priming and then painting.