There is no way to seal or expunge law enforcement records.
If you were not convicted of a crime, you have no criminal record.
A forfeiture or ordinance conviction by default is still a conviction, whether in municipal or circuit court.
Under present WI law, there is no way to go back and force expungement if it was not available and addressed at the time of sentencing. On a twenty year-old citation, even if court records still exist, it is going to be difficult to try and get sufficient information to accomplish anything by agreement, especially in a default situation.
This answer is provided for general information only. No legal advice can be given without a consult as to the specifics of the case.
Generally convictions are not able to be expunged. Contact an attorney in WI to see if you might qualify.
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There may be a way to get the conviction vacated, but that would depend on a lot of things going right. You would essentially have to get the prosecuting attorney and judge to agree to reopen the case and dismiss the charge. This may be difficult based on the age of the case. However, if you are successful, the records regarding the case will still exist, if they haven't already been destroyed, but the records will show that the case was dismissed instead of showing a conviction.