Thanks guys for the info, sounds about where I thought it would be.
I use Unger Professional cleaning fluid for widows, so not sticky like fairy, so hopefully most of my customer should be fine. I also clean all the sills and frames each time (always have done) so hopefully most of them wont be too dirty when I switch over to wfp. I will still trad some of these as suggested.
Thanks again guys for the advice, will definitely have to go down the route of RO & DI as hard water and so the resin would be way too expensive with DI only.
Cheers
Steve