Sand the surface of your fir exterior door to remove any smudges, fingerprints or other unattractive blemishes. Sand in the same direction as the grain of the wood with 180-grit sandpaper.
Wipe away your sanding debris with a damp cloth. Wipe all over both sides of your fir exterior door so you have a clean surface when it’s time to stain. Allow your door to dry completely before you apply any stain to its surface.
Hang your door to make sure it fits correctly in the doorway. Make any necessary adjustments because you don’t want to adjust it after you apply your stain. Remove the door to stain it.
Condition your fir exterior door with a wood conditioner before you apply the stain. Fir is a soft wood, just like pine and hemlock, and soft woods can appear blotchy and uneven if you stain them without applying a conditioner first. Brush wood conditioner on the door with a paintbrush. Brush in the same direction as the grain of the wood for the best results. Allow it to dry before you apply your stain.
Brush your wood stain onto the fir exterior door, moving in the same direction as the grain of the wood. Use a wood stain designed for exterior use.
Apply a coat of polyurethane to your door to help protect it against the elements. Brush it on in the same direction as the grain of the wood on your door.