Farmers or fishers exception.

Except as provided in sub. (1) (am), if on or before the first day of the 3rd month of the succeeding taxable year a farmer or a fisher files a return for the taxable year, for which estimated taxes were required on or before the 15th day of the first month of the succeeding taxable year under sub. (3), and pays in full the amount computed on the return as payable, then that payment satisfies any required estimated tax installments.