If it is old roughsawn barn siding IN VERY GOOD SHAPE I would be tempted to leave it nekkid, perhaps filling old nailholes if they are egregiously numerous or large.
If it is old roughsawn barn siding in so-so shape, or thin wood, it would probably be best to paint it but you need to know that it will be a BEAR of a job, because roughsawn lumber uses up HUGE quantities of paint and is also a pain in the butt to get thoroughly covered (unless you know someone with a sprayer system).
If it is old roughsawn barn siding in poor shape, paint won't rescue it, just let it gracefully die elsewhere (or use it for something non-structural, like a shade structure or windbreak).
As others have said, don't use it for ground-contact things (or even wet-weeds-contact or bedding-or-mud-may-pile-up-against-part-of-it contact).
Good luck, have fun,
Pat