On Wed, 01 Aug 2018 19:55:48 +0200, Thomas Prufer
wrote:

On Wed, 01 Aug 2018 17:27:40 +0100, wrote:

Thanks, to clarify I intend to first empty the galvanised steel cold
water tank and wet vacuum any scale in the bottom. I was only going
to part fill it and would add the fernox DS3 as you advise.

Fernox DS3 is OK on glavanised, I take it?

It says it's safe to use but I don't know until an expert comes along.

My guess is the acid's affinity for the carbonate is higher than that
for metal. it's moot about the zinc as that seems to have gone from
the inside of the tank.

I hope if I use the right dose it will deal with the lime scale and
leave the metals alone.

AJH