SA Fly Line Backing, DACRON, 20 lb Test, ORANGE - 100, 150, 200, 250, 300, 400, 600 up to 5000 yds Scientific Anglers Dacron Fly Line Backing is built with advanced 3M technology that allows for a small-diameter backing without losing the inherent strength of the backing material. Exceptionally thin material for more reel capacity. Low coefficient of friction minimizes wear on guides. Various lengths available wound from bulk spools of Scientific Anglers Dacron Backing. You only have to buy what you need - enough to fill one reel or buy in bulk.