Turns out: shellac mixed with some pigment makes a fairly usable marker fluid for #metalworking.
(I only had black pigment at hand, but it works equally well as the blue stuff. The ingredients were all already lying around in the shop, and I didn't want to spend the 25€ for a small bottle of the commercial stuff and wait for delivery 😉)
Final sanding still took a good while, even though I was too impatient (and tired), and only wet-sanded up to 400 grit. I focused mainly on getting the edges straight, and break all sharp corners. So there are still some scratches left, but it will probably pick up new scratches anyway during use 😅