[Update]
I have used:
1. Vinegar
2. Citric Acid
3. UHU calcium removal for bathroom glassware
4. Bleach
Doesnt seem to be any difference. I have used a sponge for the cleaning.
Wear gloves and work in a ventilated area, it's very strong stuff. Good luck!Another vote for Kilrock from the South Coast although I am going to try the Zep recommended by Lazybones51 - it's a war down here!
If you can get hydrochloric acid and soak that would 100% work. Used that to remove coralline algae at work. Best use a respiratory mask and goggles though.I have just cleaned a nice set of 3 stainless steel pans (Ikea) my wife got from local Facebay for £5. All badly scaled, as we have very hard water in our area. Kilrock, shifted most scale, but a wipe round with 10% hyrdochloric acid and pans look like new.
Dennerle cleanators are pretty good at removing limescale.