If general obligation bonds of the district for an improvement district thereof shall have been issued, the treasurer shall establish within the general obligation bond fund for the district provided for in Section 35902 a separate account for general obligation bonds of the district for each improvement district to which shall be apportioned such portion of the general obligation bond fund as is applicable to the general obligation bonds of the district for the particular improvement district. The money in any separate account shall be used for the payment of the general obligation bonds of the district for the particular improvement district and interest thereon and until the general obligation bonded indebtedness of the district therefor is discharged shall not be used for any other purpose.