Exterior wood doors have always been problematic. Wood expands and contracts because of humidity and exposure. Wood cracks along the grain or the glue lines. Cracks can be glued and clamped, but sometimes if the crack will not close up it may need to have real wood inserted to fill it in and give the glue something to adhere to.
Remove the door from the hinges with a cordless drill, leaving the hinges on the door jamb. Lay the door out in an open space on two padded sawhorses.
Run masking tape alongside the full length of cracks on both sides as close to the edge as possible.
Spread the jaws of the bar clamps over the door. Tighten the clamps. Examine the door for open cracks. Use a pencil to mark the cracks that did not close when you put pressure on the clamps. Remove the clamps.
Mix the epoxy glue. Spread the glue over the cracks, forcing the glue into the cracks with your finger until saturated.
Put the clamps back on and tighten. Insert the narrow end of the wood wedges lengthwise into the cracks that are marked. Use a hammer to gently pound them into the cracks. Let the glue dry for 24 hours. Remove the clamps.
Slide the sharp end of a chisel along the protruding edge of the wood wedges, cutting them flush with the surface of the door. Slide the chisel along all the cracks, removing any residual glue.
Sand along the cracks by hand with a small piece of sandpaper. Remove the tape. Sand the entire surface of the door by hand with 180-grit sandpaper until completely smooth, but don't sand through the existing lacquer.
Colour the edge of the wood wedge, using the stain-marker pencil. Lightly spray the door with lacquer.
You can make thin wood wedges with a sharp razor knife or a band saw and a piece of pine, or buy thin wood tape (as a substitute for wedges), brown epoxy glue and stain markers at your local home-improvement store.
Always wear safety glasses when working with wood, and always wear dust masks and or respirators when sanding or spraying lacquer.