Ellis Walentine
"Medium density overlaid" plywood is what you want. Even without paint, it will hold up in all kinds of weather. In my old shop, I had an exterior basement door made of redwood 2x4s with a 1/2" MDO skin on the outside. It never got painted, and the plywood was still in excellent shape 20 years later when the shop burned down.

MDO is more expensive than baltic birch, but it's the price you pay for the right material.

Ellis