There are three main reasons rain will leave a mark on the window after they have been cleaned. One, there is dirt or residue from the cleaner trapped in the pores of the glass that contaminate the rainwater when it sits on the window. Two, the top of the window frame is dirty and when the rain runs down onto the glass from that top frame it drags dirt down with it. Three, the rain has fallen through heavily polluted air, trapping contaminants in the droplets on its way down. The last one you can do nothing about, but it is also the least common problem. The other two problems, you can do something. I would say it would be advisable to carry around in your car or van a low residue cleaner, like a bottle of GG3/4, even if you don't use it on a regular basis, just to use on damp days. If your customer looks annoyed or says something, explain to them that you use a different cleaning solution on rainy days that helps prevents rain marks, which is true. If they are still unsure, just ask them to let you try it once and let them see the results. When you carry out this type of clean, make sure you thoroughly clean the top frame, and give the glass a really good clean as well. If you use that method with a low residue soap you will almost certainly never get rain marks on the window within a few days of them being cleaned. It may be slower to work like that and to use GG3/4, but it's better than not working at all. Over time your customers will become re-educated, and will realise that you really do know more about your job then they do
