At any time after entering a contract for a project to be financed in whole or in part by special assessments, the district may issue temporary and definitive bonds on the project fund created for that purpose in the manner and subject to the limitations prescribed in section 40-24-19. If the bonds are issued to finance an irrigation project, the net revenues derived from the imposition of service charges to be imposed and collected with respect to the project as provided in section 61-24.8-22 may be pledged to payment of those bonds. Bonds issued under this section must be in amounts as in the judgment of the board will be necessary for the project. The bonds must bear interest at a rate or rates and be sold at a price resulting in an average net interest cost not exceeding twelve percent per annum if sold at private sale. There is no interest rate ceiling on bond issues sold at public sale or to the state or any of its agencies or instrumentalities. The bonds must state upon their face the purpose for which they are issued and the project fund from which they are payable and must be signed by the manual or facsimile signature of the chairman of the district board and countersigned by the manual or facsimile signature of the secretary of the district. The bonds must be payable in such amounts as the board determines, extending over a period of not more than thirty years.