1. Do the work in spring or fall, if possible. Wood trim is best installed before moist summer air makes it swell or dry winter air causes it to shrink.
2. Acclimate the wood indoors. Ideally, its moisture content should be 8 percent when measured with a moisture meter.
3. Prime all sides. Seal every face, edge, and end of every piece with an oil-based primer to protect the wood from water and reduce seasonal swelling and shrinking.
4. Paint the tongues. Before installing the tongue-and-groove boards, paint the tongues the same color that you will be painting the finished wainscot. That way no other color will show through when the boards shrink in the winter.
5. Brush alongside joints, not across them. Any paint film that bridges the joints will crack as the wood moves.