For the payment of the principal of and interest on all bonds issued pursuant to this chapter, as they respectively mature, and for the creation of the sinking fund as may be necessary for the fund, the full faith, credit, and taxing power of the county must be irrevocably pledged, and there must be levied annually by the county auditor and collected by the county treasurer a tax sufficient to pay the principal of and interest on the bonds as they respectively mature and to create such sinking fund as may be necessary; provided, however, that in the event that such revenues from the ad valorem tax levied in the service areas under the provisions of item (1) of § 4-19-10 or from the rates and charges for fire protection services must be available for the payment of debt service on such bonds (whether or not such revenues have been pledged for that purpose), and must be delivered to the county treasurer for the payment of such principal and interest and for no other purpose, prior to the occasion when the county auditor fixes the annual tax levy, and annual ad valorem tax to be levied for the payment of the principal and interest on such bonds may be reduced in each year by the amount of such revenues derived from such taxes levied in the service areas or from such rates and charges which are actually in the hands of the county treasurer at the time the tax for the year is required to be levied; provided, further, that bonds issued for a service area subject to the imposition of taxes must be primarily the obligation of the service area and for the payment of principal and interest thereof, as the same mature, there must be levied and collected service charges, assessments, or ad valorem taxes upon all taxable property in the service area, and resort to the tax levy required by the preceding paragraph of this section must be made only in the event that funds from the sources required by this paragraph prove insufficient to meet the payment of the principal and interest.