This has been said by the more knowledgeable posters than me, but to summarize, a "foul ball" can never become fair. But a ball spinning about outside the baseline is not a "foul ball" until it is touched or comes to rest. That's why F5s and F3s are taught to swipe the ball if it's outside the line, thus making the ball FOUL.

Unless of course you're in the Major Leagues....

Well that's because in the Major Leagues it would be field interference!!!