If you are going to paint the rears, you need to strip them back to bare metal otherwise any imperfections, rust or other issues with the current coating will eventually come through and ruin your new silver finish. The middle parts of the rears (the bit with the piston in) is made of aluminium, so you could strip it back and just coat it was some high temperature clear lacquer. The outer part that includes the mounting braket that the pads sit in however is steel so even cleaned up the metal isn't quite so attractive, so lacquering it would be like putting lipstick on a pig. That said, I quite like the idea of silver calipers as I think the usual red is a bit loud.