The Best Way to Finish a Knotty Alder Exterior Door to Preserve Color

Select a clear water-repellant wood preservative, which contains fungicide to prevent mold, wax to repel water and solvent and oil to penetrate and preserve the wood grain. Look for the temperature range on the container. Water-repellent wood preservatives only work in a certain temperature range, if it is too hot or too cold, the finish won't dry properly.

Wait until the temperatures are right or take the door into a workshop or garage where you can control the temperature range.

Spread a drop cloth under the work area. If the knotty alder exterior door is already in place, apply the finish there. If it is not installed already, put the door on saw horses or lean it upright against a wall.

Brush off dust and dirt from the wood surface using a soft clean rag. If the door is new, you can apply the finish directly. If the door has been previously finished, sand the surface lightly first with an 80-grit sandpaper then with a 150-grit sandpaper. Wear a protective mask when sanding surfaces.

Shake the container of water-repellent wood preservative to thoroughly mix it before removing the lid.

Brush wood finish onto the knotty alder exterior door with a soft-bristled synthetic paint brush. Apply the finish evenly and do not let it pool or drip on any part of the surface. Take extra care to thoroughly coat the edges and corners of the door.

Allow the finish to dry for the full duration of time as specified on the container. Drying times vary depending on the product.