If left exposed to the elements, your bare wood doors will warp and turn an ugly weathered grey. The proper finish will seal the exterior door and inhibit premature wear and decay. Choose a particular type of finish based on the condition of the wood and your personal taste.
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If your doors are already stained, choose a type of polyurethane wood sealer that's based on the type of stain you're working with. Add a solvent-based polyurethane to exterior doors coated with oil-based stain. If the doors are finished with a water-based stain, use a water-based polyurethane. Choose the correct sealer, or cracking and flaking could follow.
Bare Wood Doors
You can finish bare wood doors with either a water-based or oil-based polyurethane. Solvent-based polyurethane tends to penetrate deeper into wood fibres. This trait causes them to endure better. Water-based polyurethanes are easier to work with and less toxic.
You can also seal an exterior door by adding a painted finish. An exterior acrylic paint will preserve the wood and add a colourful finish. Be sure to add an acrylic primer to the door prior to painting, or the finish will peel.