The rule is that a prefix is added to the front of a word to make a solid word.
However, when there is already a word that has a different meaning, the prefix should be hyphenated. This usually occurs with the prefix re-.
…He decided the only choice was to resign.
…We returned to the bank to re-sign the papers.
…He was looking for redress for the grievance.
…Her mother demanded that she re-dress for the more formal occasion.
And here is a really fun one.
…The decision was made that the company should be unionized.
…It was an un-ionized element.