Hello KNO3,
also from me a thank you for your contribution. I am familiar with the Bruennier agent from Regner. I have not used it so far because the blueing effect of the antique machines is not achieved so well with it in my opinion.
The Regner kettles are usually all blackened. A trademark of the company. The blackened boilers and the machines look very good. I like it. But with the old machines, the effect looks different.
But my thoughts are also very theoretical and I am happy to be proven wrong by practice.
Stay Healthy