You may have been taught that you shouldn't use done to mean "finished," but it's not that simple.
When you push back from the Thanksgiving table and say, "I'm done," a cranky relative may attempt to correct you by replying, "A turkey is done; you're finished."
Although done has been used to mean "finished" for centuries, admonitions against it started surfacing in the early 1900s. The first style guide that advised against using done to mean "finished” didn't give a reason for the declaration, and the current Merriam-Webster Dictionary of English Usage speculates that the advice was based on bias against the usage's "Irish, Scots and U.S." origin.
The "rule" against done has been widely taught in schools, but no historical pattern or logic supports it, and most credible modern usage guides either don’t address it at all (e.g., AP Stylebook, Chicago Manual of Style, The Cambridge Guide to English Usage) or simply note that done and finished are interchangeable (e.g., Garner’s Modern American Usage, Merriam-Webster Dictionary of English Usage, The Columbia Guide to Standard American English).
Take that, Aunt Ruth!