I use WFP mostly and am having my van sign written.
What do you suggest I have on it?
How about:
'ABC Window Cleaning.
Phone 01291 621951 for a no obligation quote.
Ask for Tosh'
Seriously though, I don't think you need to specify anything at all about pure water cleaning. I honestly don't think the customer cares as long as their windows are left clean; you're a nice guy and provide a good service.
Why 'bamboozle' potential customers with geeky window cleaning technicalities? You can do that when you turn up to clean; by which time it'll be too late for them to refuse.
Look at other trade signage, sometimes I'm confused to what they do or what services they're offering. That must be a waste of time, effort and cash on naff signage.
I reckon if you keep it simple, you can't go far wrong.