That's what happened: Last week the oxalic acid crystals arrived from ebay. I got to it today, using the ratio that Camille1 kindly provided. Brushed it on, scrubbing vigorously away, waiting patiently for (at least) 25 seconds and - nothing happened.
Frankly, I had my doubts, it simply sounded too good to be true. Oh hell, since I had mixed it up already I might just as well brush on the rest of it, might at least do some good. Once finished I turned back to where I started and couldn't believe my eyes! Gone was the rust, all gone as if it never had been there! This stuff really works wonders! See pictures. Since I was at it I used it on some stainless parts
as well. Worked miraculously, all shiny new! From now on it is not only barkeepers friend, but mine as well. Thank you all who pointed me in the right direction!