You can't fix it. The coin is worth 10-20 cents with no date, so there's no harm in finding out the date, but you are etching the coin with vinegar and removing some of the material. Occasionally, you can sell some of the very rare dates that are "revealed," but mostly they are good just to fill in a hole in your own collection. I have many rare dates I've gotten from doing this, and they fill in holes in my collection, but they still aren't worth much. Most collectors don't want "revealed" coins no matter how rare they are.