Backing or dacron, is a braided yarn, in cotton or polyester that is used as a filler for fly fishing reels. There are various types, thickness and seal, from the thinnest and most resistant to the backings with lower capacity.
A 20Lbs backing is normally mounted on a one-hand fly fishing reel, it is very difficult that this is used over time, therefore the seal will be indicative and not fundamental.
It is not the same thing if I practice big predator fishing as can be sea or salmon fishing, in this case, backing is important. It will have to have a greater seal and take up little space on the reel, for this type of backing the best solution most of the time is to use a good quality spinning braid, this will certainly be thinner and more resistant than any backing on the market.
When a rat tail is mounted on a reel, to make a correct filling, I will have to follow a series of steps:
1-mount the mouse tail first and then the backing, in this way I can understand how much filling I have to do
2-wrap everything on an empty reel starting from the backing, the mouse tail head will be free
3-wind everything on another empty coil, in this way I will turn the windings over, I will have the backing head free
4-wrap everything on the reel's reel.