On 06/10/2020 15:50, www.GymRatZ.co.uk wrote:
On 06/10/2020 15:43, Jethro_uk wrote:

You need hydrochloric acid. Probably the strongest supermarket cleaner
with highest concentration of HCl is Harpic 10x, about a quid a bottle.
The stain is basically discoloured calcium carbonate.

Could it not be removed mechanically by scouring ?

If it's limescale it'll be rock solid. You might chip it off but if
there's scale you can see it's going to be just as bad on the blind side
that you can't get to.
Best off with a full de-coke as required.


what about kettle descaler and a kettleful of hot water?