Having cleaned my windows without any problem for more years than I can care to remember, we have just noticed that each of my new'ish double glazed units have the odd slight mark following a clean on Sunday. More than a mark, more a light scratch that is visbile when the sun shines. I've seen posts on using cerium oxide and the like but I've also heard that toothpaste will do the job (!). Has anybody tried this or can explain how it works? Just off to get some more Colgate.....