It has been awhile (2015!) since I have posted on this page. The cool thing is that the weather has been nice enough to work outside on the garage. We used four 3/8 plywood sheets and stained them with the same stain as the rest of the wood exterior. Here they are set out so we can match grain a little and check for blotches.
We used clamps to hold the 21 x 96 inch panels in place on the first few boards.
Here is a shot with five panels in place. Once we had the bottom row up, the rest of the layers could rest on the boards below so the clamps weren't needed.
Here is the finished project. We got the idea to cover a standard steel garage door with plywood while on the MN Architecture Tour a few years ago.
Here is a closeup of how we fastened the plywood to the door. We used 2 inch Allen-button-head machine screws. They are stainless steel of course so that they don't rust.