Hi Lewis
We tried salt to give extra slip as we live in an ultra hard water area and like you, we found you get white lines that over a few weeks become even more visible.
It's the same type of salt that gives us problems as all our work is near the coast. I then though if we get problems with salt on windows anyway, and we're adding salt to get more slip, then it's like putting petrol on a fire to put it out.
So that idea went out the window so to speak.
After going through a bottle of Fairy or more a day to get more slip, we found that left us with other problems, namely the glass ended up sticky and detailing became a problem too.
I tried Dai's idea, adding rinse aid. It works but not with regards slip though - it only gives a brilliant shine to the glass and we buy ours from a local pound shop. It don't give much extra slip but boy oh boy, does the glass shine.
As for slip, I now accept that around here that getting good slip without spending a small fortune on GG4 or Squiggie Off doesn't happen.
I hate to say it, but it comes down to both technique and having to wipe off the odd the odd place where the squeegie sticks.
With some houses I now go over the glass every other month with wire wool and so far, that is the best option I've found. If you use a big wad of it, it takes seconds.
I beginning to think that every property is unique for cleaning.
cheers
Ross