Metal exterior storm doors are uninviting and do not allow natural light to enter your home. Installing a window in the exterior metal storm door will create a welcoming front entry to your home. Metal doors are comprised of two steel panels with insulation sandwiched in the middle. There are wood door supports within the door that add additional rigidity. The window size cannot interfere with the structural integrity of the metal door and must be sized accordingly. Cutting the hole for the window in the metal door is slightly more complicated then a wood door, but simple with the proper tools.
Place a punch at the base of the lowest hinge pin on the door. The hinge pin's base is the end that does not have a flat head. The base of the pin will be recessed and generally on the bottom of the hinge. Hit the end of the punch with the hammer to remove the hinge pin. Repeat the process to remove each pin. When you remove the top hinge pin, support the door or it will fall and get damaged.
Lay the door flat on across some sawhorses. This will give you a suitable working platform to mark and cut the opening for the window.
Mark the dimensions for the window per the manufacturer's rough opening size recommendations. Measure the width of the door. Divide the dimensions by 2 to find the center of the door. Mark the centerline of the door near the door's top edge, using a carpenter's pencil. Place another mark near the bottom edge of the door. Draw a straight line connecting the two marks, using a straightedge. Determine the height of the window. Mark the top height of the window on the door, at the centerline. Set the framing square even with the height mark and straight on the centerline. Draw a line out from the centerline to the outside vertical edge of the door. Flip the framing square and extend the line in the opposite direction.
Lay out the width and height of the window rough opening per the window manufacturer's installation instructions. Mark the recommended width on the door. Measure down from the height mark on the door and place a mark with the carpenter's pencil. Extend the mark out both directions from the centerline. Divide the rough opening width dimension by 2. Measure off the door centerline, near the top of the rough opening, half the width dimension and place a mark. Perform the same process on the other side of the centerline. Place two additional marks near the bottom of the rough opening.
Drill a 1/8-inch hole at each corner of the window rough opening, going all the way through the door. Flip the door over. Redraw the rough opening by connecting the 1/8-inch holes with the straightedge and the carpenter's pencil.
Enlarge the 1/8-inch hole by redrilling the hole with a 7/16-inch drill bit. Insert the double-cut attachment into the drill. Align one edge of the double-cut blade on the pencil line and the other edge of the blade will be inside of the rough opening. Tilt the drill toward you and apply pressure forward. When the line is cut, repeat the process until all the lines on that side of the door are cut. Insert the knife into the cuts to free the insulation. Flip the door. Repeat the cutting process.
Install the window per the manufacturer's installation instructions. Rehang the door and insert the hinge pins. Caulk the window frame in the metal door. Open and close the door to ensure that it is functioning properly. Prime and paint the door.