To make 1/2" plywood outdoor-tolerant, cover it with tarpaper or
Tyvek (housewrap) and put siding on it. The intermediate layer keeps
moisture in the siding from wicking into the plywood, and the siding
takes the weather (rain, wind, sunlight, ivy, woodpeckers) until you get tired
of how shabby it looks and replace it.

Paint-on-wood is less appealing than other siding choices.