i did a conny lid the other day and he wanted the windows on done too. so, i washed it down with tapwater with ecover squirted in, and while at it went over the windows with it to get the thick off. anyway switched for pure to do them proper, but half of them did'nt look like they needed it! naturally i had a bee in my bonnet so i've had a go at some of my windows with just tapwater or tapwater with ecover. on the patio they looked as they should-dismal! but on the shed french doors they were ***h on perfect, just a few spots round the edges where water beads, thus, i theorise that on some windows it really does'nt matter what the tds is (ie glass that sheets) if only you knew exactly which ones! incidently my tds was around 200 today.