white spirit and turps,will break down the polymers in pvc ,so constant use will rapidly degrade your windows,
the best and safest way ive come across (ooooh errrrr! ) has to be johnstones baby oil on a cloth ,wipe on wipe off,makes the window waterproof,and water beads on them ,pluss the grit and grime can be hosed off in the future with ease, pm me to tell me how you got on