Manually spooling 300m of backing off and back onto a fly reel after every saltwater trip is painful. But essential to prevent corrosion. A fence wire spooler and a cordless drill offers an easy solution.
 
Fencing spools come in a variety of diameters. This 30 cm version gives you almost a meter per crank and has a built in sturdy stand.  Wooden dowels as struts give a large circumference.

Duct tape prevents the backing from wrapping into the joins

 

This fence spool had the option of a drill attachment. At 0.9m per turn beware. If you are missing the sound of
a screaming drag, it is easy to replicate a sailfish on steroids. I take no responsibility for any burnt out drags, mangled fingers or smashed reels. Use the slow setting and set the drill clutch.
150m of 60lb power pro barely makes a dent on the capacity – still lying in a single layer of strands. This makes rinsing the backing and drying it very simple. When the backing is lying several layers thick and under tension it takes a bit more soaking to get rid of the salt. I take the spool off the stand, give the backing a rinse under the shower and then leave to soak in the sink, with 2 or 3 batches of fresh water. I rinse off and let it dry. 
The drill attachment can also connect to your reel. I have tried a few ways, but this method with foam gives a smooth hand held operation and allows you to evenly spool the backing.  Foam also means that there is  no risk of scratching your reel.  If you do not centre your reel, forget about a drill. As each reel is a different size, I created a base template with a central gap for the reel handle and counter balance. And then simply pin on a foam template suitable for the reel. Make sure the pins are outside of the spool diameter to prevent scratching. 
I still don’t have a tensioner for the big fencing spool. But as you are loading from a big spool onto a small one, I find that my foot is able to provide more than enough tension for braid and you get a very even load.
The foam setup is also great for winding fly lines on and off.  A bit rough around the edges, but does the trick.