Being a chemist... Just wondering what the metal is that they are made out of? Have you looked into making a clean coat with a stain, it is called anodizing. You can anodize many metal parts, not sure if people on here have tried it or if it would ruin your brakes...

 

I use hammerite red smooth and I do it in warm weather or use a hair dryer on it to help bake each layer on I did 4 coats and same as others it holds up fine I will re do in summer but lasted 3 years and only redoing as want perfect after new disks n pads

 

Bavarian sell brake caliper covers!!!