In message , The Natural
Philosopher writes
Andrew Gabriel wrote:

If it's thick brown and not clearing, then you probably
have a layer of organic matter protecting the limescale.
To clear this, use dishwasher detergent dissolved in hot
water in the same way. Don't mix the two chemicals
together though -- alternate treatments if necessary.


I usually have started with strong caustic to remove organics, and then
acid to attack the scale.

I wasn't aware you could buy sulphamic in 'pure' form cheaply. If so,
then I withdraw the brick acid recommendation in favour of sulphamic.
Its a good descaler.

I've got a 20 kilo drum of it at work

--
geoff