A double-walled fiberglass reinforced resin underground storage tank including a generally cylindrical inner wall having a plurality of circumferential support ribs bonded thereto, at least one spacer filament mounted on the outer surface of the inner wall of the tank over a substantial portion of the length of the tank, and an outer wall substantially conforming to the contour of the inner wall and the support ribs but not adhered to the inner wall or the support ribs. The outer wall cures into relatively tight conformance with the inner wall and support ribs resulting in a reduced annular space between the walls. Small voids are created immediately adjacent to the spacer filaments, providing fluid passageways which enhance the otherwise slow migration of fluid in the annular space.