I had a custy with a patio door that has had terrible limescale for several years before I turned up. Between them and me we tried limescale removers, vinegar, household cleaners, all to no avail.
Then someone on here told me to use a Brillo pad. I used it on an inconspicuous area, making sure the soap content didn't run out and the result was perfection. The customer finished off and is overjoyed.
Vin