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Unger rub out is what I have used with good results bit of elbow grease but works good, I do a gym in someone's garden floor to ceiling windows and the owner used to leave the sprinkler on splash all over the glass not no more he now uses the hose end to water close to the gym.
Owt that removes limescale seems to work although like Den says you still need a bit of elbow grease, I also find that using unger rub out or Viakal or such like with a magic sponge seems to make it a bit easier. Other than that Greens suggestion was a potato or a lemon cut in half - which I've still not tried!!!