I de-rusted the calipers as much as possible then I boiled it with soda.
I made it wet and treated it with fosfor acid a few times.
Then some light sanding and sprayed the calipers 3x with 20 min pause.
The result is very good as seen on the pics.
I tested it in Grey and in black, and made my choice in grey metal.
The cat is guarding my old calipers.
They will be prepared for spare