I must admit, this tip was taken with a pinch of salt - as the substance recom-
mended by this modeller was "toothpaste".
You simply rub a small amount of toothpaste, applied using finger, on the window surfaces and allow a few minutes, then wash off toothpaste with luke warm clean water.
Then wipe dry with a lint free cloth, leaving a nice clean window surface.
Anyone in doubt, can always try this out on an old coach no longer being used.