I've used that blade, but found it a little to big and it didn't maintain good contact with the glass.
Unger do one similar to that I think? It may even have been an Unger one I used
But Unger also do a stainless steel, fold back style knife, this one is top banana, it has a replacable razor blade, and when folded back on itself it can be clipped into a standard squeegee handle (instead of the channel of course :
)
Ettore used to do a brass one that did the same, but the Unger one is better (or was better, I haven't seen the Ettore equivalent for many years now)
So for major initial cleans its first rate. And of course you also use it just in your hand as against in a squeegee handle.
I know you can use a standard stanley scraper, but they are better in the hands of the decorators for tidying up their cutting in.
Stanley also do a 2 inch retractable razor style scraper, ideal to keep in your pocket or pouch for the odd bit of nibbing off, and they are top banana on the ceramic hob
Under powered for initial cleaning though.
Technique is much as windows_chepstow said, soak up up & knife off.
Warn whoever you are doing it for that if the builders have left sand & cement on the glass you can't guarantee there will be no scratches.
These razor knives won't scratch the glass, but sand & cement (compo) certainly can.
Even go so far as to have a close look at several of the windows and check for scratches, and then point them out to whoever is paying for the job.
It's a sod to defend yourself if after you have got the windows all nice and sparkly, customer then see's lots of scratches and blames you, claiming they weren't there before you started
They almost certainly would have been, but the glass will have been so dusty/dirty/grubby that they won't have noticed any scratches that were there.
DON'T GET CAUGHT OUT!!!
If like me you are a WFP user, tough
for a builders initial clean you have to get up close and personal and do it the traditional way
Ian