A hole in a garage door allows airborne contaminants to enter the garage and so it must be closed. You can patch a hole in your garage door without having to call in a professional; the procedure is straightforward if a bit involved. You will need tools that most homes already have on hand, along with supplies from a hardware store. The structural integrity of the garage door will be unaffected by your patching the hole.
Place plastic tarps on the ground beneath the hole on both sides of the garage door. Turn off the garage door opener, if there is one, so that the door will not move while you are patching the hole.
Working from inside the garage, smooth the inside edges of the hole with rough-grit sandpaper. Blow off the loose dust generated by the sanding with bursts of compressed air.
Shake the can of fast expanding, spray foam insulation for ten seconds. Spray the insulating foam into the hole. Stop spraying the foam when the hole is completely filled.
From outside the garage, press the edge of a wooden painting stick against the garage door at one end of the hole. Press down on the stick as you drag it across the door to smooth the foam even with the door. Repeat this procedure as many times as necessary.
Go back into the garage and repeat the procedure just finished at the hole. Let the foam set for the amount of time specified in the instructions that accompany it.
Sand the foam-covered hole on the outside of the garage door with rough-grit sandpaper. Stop sanding when the surface of the foam-covered hole is even with the rest of the door. Repeat this procedure on the inside of the garage door. The surface of the garage door can now be painted to match the rest of the door; alternately you can paint the entire garage door on both sides with a fresh coat of paint.