im a window cleaner so ill clean dirt,moss,algae,bird muck,pollen spots,etc from their frames,glass and sills but render/hard water stains/paint on frames etc NO.
for one window its not worth faffing about for (and ordering nasty corrosive chemicals that may/may not remove it).
why are you even worrying about it?id say im sorry but i cant remove it.they wont dump you just because you cant fix someone elses f*** up.
99.99% of windows are fine so dont worry about the 00.01%