I have actually used it on limescale where an overflow had been dripping and splashing on a patio door.
First,.. dilute it 2:1
wear goggles, face mask, gloves etc,.. even diluted its not nice stuff.
Don't get it on any stainless steel, cement or natural stone as it will cause damage very quickly.
Use a dishwash sponge to apply it and leave for a few minutes, then clean off with 0000 wire wool. Rinse the whole area with clean water. Job done.