Simon
90% of our work are Builders Cleans. Virtually all the glass we clean that has stickers, paint on etc, we clean using a 4/6" Ettore Champion scraper or similar.
As J V has said, wet glass well and always use a fresh blade.
The trouble with using white spirit/thinners is that some adhesive can become 'smeared' across the glass and harder to remove!
The 'film' that J V mentions is, I think, the 'anti-glare' stuff that is usually found on the inside of some windows, i.e.......easily 'torn'!
Regards.....Jake......Westclean