Not seen it to that level before but once the paint starts peeling doesn't take long for the weather etc to get to them. I'd get them rubbed down and repainted with something like hammerite (I've just stripped to bare metal and painted a spare set of calipers for mine and didn't take too much effort). Or If you've got the funds refurbishment or brake upgrade! Also with where the bleed nipples are on the rear calipers its easy to spill brake fluid on them if a correct tool isn't used, which quite easily peels the paint if not cleaned off correctly.