Here is a household product with which I have become acquainted:
I started with a spare Zentih mower carburetter I had knocking around:
I attacked one of these with Cillit Bang descaler and got this in a few minutes:
But after a couple of hours with a small brush & Cillit Bang:
This is what it is supposed to look like:
The next step is to make some decision on how to plate the steel parts. I fancy a nice yellow chromate passivate...