A coatable insulator composition for use in the formation of a green tape comprising fine inorganic micropowders composed of amorphous glass and a refractory oxide in a volatile organic solvent solution of a binder composed of a polymer substance and a plasticizer(s). A green tape obtained by coating said composition over a flexible substrate, and heating and drying this product to remove the organic solvent; and a method for forming a barrier-rib for a plasma display apparatus by sandblast etching using said green tape as an insulator layer for the barrier-rib formation. A use of a pre-formed green tape of uniform thickness in the formation of the barrier-rib of a plasma display apparatus allows the thickness uniformity of the barrier-rib to be improved and the work rationalized, which means that the plasma display apparatus can be made larger and finer, and mass production throughput will be enhanced.