Revenue from and any amount due under any annual contributions contracts pertaining to such project or combined projects so purchased, acquired, constructed, improved, enlarged, extended or repaired, shall be and remain subject to the statutory lien until the payment in full of the principal of and interest upon the bonds. The holder or holders of the bonds or coupons representing in the aggregate not less than 20% of the entire issue then outstanding may, either at law or in equity, by suit, action, mandamus or other proceeding, protect and enforce such statutory lien and may, by suit, action, mandamus or other proceeding enforce and compel performance of all duties of the officials of the borrower, including the fixing of sufficient rents, the collection of revenues, the proper segregation of the revenues of the project or combined projects and the proper application thereof. However, such statutory lien shall not be construed to give any holder or owner of any bond or coupon authority to compel the sale of such project or combined projects or any part thereof.