I have couple of windows on a house i started cleaning last month and i have noticed some marks on the glass that look like water marks.
They are very hard to remove, i can only put it down to some sand or lime stone slabs that are above them on the mini wall that surrounds a flat roof, so when it rains the water from the slabs drips onto them.
This has properbly been going on for some time now and wondered if there is any thing i could use to remove the marks