Not Viakal - it will turn upvc yellow in time, as will most limescale removers, in my experience.
Nightmare time Gaz.
I have done plenty in my time. I really try to avoid them now. the thing is, the only thing I found really works is the old trad way. Loads of soapy water on your applicator, let it really soak the window, do as many as you can get to, go back soak them again, and scrape with fresh blade, soak again, squeegee off.
Slow tedious job.
Trouble really comes because the same chemicals used in processing sand and cement are used in manufacturing glass, so the cement mix can get pitted into the glass, then it is impossible to remove completely. This has to be clearly explained to whoever is taking over the site. When I have had to face that situation, which I have a few times, I have got the manager concerned to physically see what I do and view the result, explaining as I go. They see for themselves it is not your fault or problem. Then you get the go ahead, and they accept whatever the best results you can produce.
I will only do a build clean now, if I'm going to get the regular contract. And initial clean has to be at least double.