Sand the exterior door lightly with 220-grit sandpaper. Work the sandpaper over the door to gently roughen its surface.
Wipe down the door with a damp cloth to remove any dirt, grime and dust. Allow the door to dry completely.
Wet a cloth with paint thinner and wipe down the entire surface to remove any remaining sealant. Wear a mask, rubber gloves and safety goggles when handling paint thinner.
Wet a paintbrush with polyurethane sealant and apply it to the door using straight, back-and-forth motions. Continue to apply a single layer of the sealant to the door, paying attention to cover the nooks and crannies.
Allow the sealant to dry for at least 24 hours before applying a second coat of sealant.