An often-overlooked step can keep a new wood door from binding over time: Seal the bottom and top edges with polyurethane or paint. Coating the edges prevents the wood grain from absorbing moisture in humid weather and swelling or even warping. This technique holds for both exterior and interior wood doors, and also for existing doors that have been cut to clear a carpet. Because the surface isn't visible, you don't need an expensive brush — a cheap bristle or foam brush is fine. Apply two coats. After the first is dry, sand lightly with fine-grit sandpaper or steel wool to smooth the surface before putting on the second coat.