I've an idea that might stop the windows that bead,... well beading really. Its early days yet,.. but my thinking would be that if the beads of water are either a lot smaller, less of them, or even gone all together,.. then there would be nothing left to cause the spots when it dries out???
It would involve adding a chemical to the water,... a very small amount hopefully,.. but I know already that there will be many doubters who don't want to "contaminate" water that they've put so much effort into purifying.
I've a few different chemicals to try,.. to see if any of them are half as effective as I'd like, and i'll be looking for something that will lower the freezing point of the water too,.. so hopefully it will be a dual purpose solution.