IMHO, "Exterior" plywood is just another misused term in the lumber world. Most of it will not withstand direct exposure to the elements. If it is CDX, it probably won't last more than one season as yard ornaments. If BCX or ACX, your odds are a lot better, but it will need very good sealing with an oil-based coating. If you read the fine print, these are rated for exterior construction, but require covering with something else (i,e tar paper/shingles, siding, vinyl siding with plastic wrap, etc). Sheets rated as Exterior Siding (i.e RB&B, T-11, etc) will hold up well if you seal the edges and any fasteners. These are made to survive the elements with a good paint coating providing the edges are sealed, or not directly exposed to rain, etc. Pay particular attention to the edge that rests on the ground.
Oil-based Kilz (not Kilz II which is water based), is a good coating to seal it, but it will take at least 2 and probably 3 or 4 coats on the edges, followed by at least one topcoat.
If using oil based, make sure you cover all sides of the wood. Otherwise, moisture will get into the wood from the back side and cause the oil-based coating to lift off (flake and peel). If using exterior siding, you can use latex, but be very generous when applying it to the edges. It still won't tolerate a lot of ground contact, to raising them up some would be beneficial.
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