If your library is made of wood, then think of it like an exterior deck. Raw wood needs to be painted or stained to protect it from water and sun damage. Standard exterior house paints are a great option. They’re durable, inexpensive, and universally available. We recommend applying a coat of primer before painting or staining your library; the primer is designed to bond well with wood, and will help your paint or stain adhere better. Apply more than one coat of paint or stain for extra weather protection, and reapply every 1 – 2 years, or as needed.
Some people choose to add a few coats of polyurethane varnish on top of the paint or stain, but it may not be necessary. If you’ve painted your library with house paint, that’s more durable than varnish. Applying a new coat of paint every few years will likely protect your library better than varnish will.