Is the plywood treated? If so it should last awhile depending how old the plywood is and how much element exposure it has seen.
As far as paint goes, I would not even consider it. You will be sorry when it flakes off. I would look into a good stain with some pigment to it for sun protection. Myself I like the Duckback products. I have used their Super Decks stain for years on my deck and fencing. It soaks in like an oil and is a one coat application. It has good wood preservatives in it and it doesn't flake, just fades over time.
Thompsons water seal, you get this in a Golden tint (premixed) as well, this works well & with little hassle & as journeyman said, definitely do not paint.-Mezz