GB, interestingly, had job yesterday where wall has just been rendered (last week) custy cleaned off cement from french windows (2 sets). We went yesterday, both sets of windows were a real mess, runs from builders rendering. Thought about scraping but would of taken ages, then remembered, Evans Hi-Phos which I had. Popped some on micro-fibre cloth and wiped over glass. Easy as that, all gone. Then trad cleaned.
Hope this helps,
Peter