Bonds may be issued under the provisions of Sections 4-49-1 through 4-49-21 N.M. Stat. Ann. for the purpose of remodeling and making additions to necessary public buildings and for water, sewer or sanitary landfill systems and airports under the same conditions as provided for issuance of bonds for constructing them. Provided, however, that no money derived from general obligation bonds issued and sold under the provisions of Sections 4-49-1 through 4-49-21 N.M. Stat. Ann. shall be used for maintaining existing buildings, and, if used for that purpose, the bonds shall be invalid.