use a sodium hydroxide soloution in a squeezy bottle. Spray on dry and let it do its work.and wfp off in 5 mins I charge up to £20per window.
Had a customer ask me not to come this time as they were going to replace the badly scaled up kitchen window with a new glass unit. I said if i can remove the scale for an extra £20 and make it look as new would she consider? Id even said to her if the results arnt good forget the charge. She said ok..
Turns out that it saved her £300 on a new unit, only took my a few moments results were amazing. Since then i have done 4 other windows like this for £20 a pop. You have to do a perfect job though for the price.