Dutch doors are a great way to provide ventilation to your house while still keeping pets outside. Most solid-core doors can be converted by cutting them in half with a circular saw and adding trim, latches and hinges. The trim will prevent the doors from overswinging and also act as weatherproofing. It's a good idea to buy a whole new set of door hardware, including an extra set of hinges, for an integrated look.
Take the door off its hinges and remove the doorknob, latch and hinges. Fill the holes with putty, then let dry and sand smooth. Install four new hinges equidistant from each other, rehang the door and check that it swings smoothly.
Remove the door again and set it on two sawhorses. Use a straight edge to make two lines a quarter-inch apart midway between the two sets of hinges. Cut along both lines with a circular saw.
Use the router to rebate a three-eighths-inch groove about a quarter-inch deep on the top interior edge of the bottom door. Cut out a three-eighths by three-eighths-inch piece of wood as long as the door is wide and affix it to the interior bottom edge of the top door with glue and finish nails. When the glue dries, round this piece with sandpaper. It will function as a door stop.
Rehang the door and affix a piece of trim to the top exterior edge of the bottom door with glue and finish nails. Do this with the door closed to be sure the trim completely covers the gap between the doors. Seal around the trim with caulk.
Paint the door, then install the new doorknob, deadbolt and latch.
Add weatherstripping to the rebated groove on the bottom door for extra insulation.
Use care when working with circular saws and routers. Never put your hand in the path of a moving saw blade or router bit. Wear goggles when working with these tools.