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IMO you are better off leaving the plywood exposed. Reason: it will get wet either way, honestly, tarps are just not very effective at that sort of thing, but if it is left open it will dry out well in between wettings. (Plus, it will not get wet very MUCH this time of year, as most of yer precip right now is in the form of snow
) Whereas under a tarp it will tend to stay damp, never a good thing.
Then when you get a dry warmish day in March or April you can paint or whatever, properly. (There are solvent-based paints that can be applied down to 0 F i.e. -18 C... but I do not know of any *clear* things that can be applied that cold, although there may well be something I don't know about).
Honestly, it is not something to freak out about -- worst case scenario it may grey a bit before you get to painting it, but such is life. And, it might not, depending on when you can do it.
Good luck, have fun,
Pat