hey mate how did you clean behine the glass balcony??? 
quite simple mark you can't. there were some dirty ones where the flats are still empty they will be cleaned internally by the builders.
all we can do is let the water cascade from the top bar when they have been cleaned
hopefully they will stay OK.
there's always bits on any big build that you know you just can not do so you improvise and hope it,ll be OK.