I did mine over the weekend with the metalcast, i have 62k miles and my calipers where pretty dark. Cleaned with brake parts cleaner, and mineral spirits, and they were still dark. Painted the were pretty dark, but didnt look bad. I sanded a hidden part and it made a HUGE differance (positive), but they also make a special primer for plastic and other surfaces that might work better (ie. easier/faster) then sanding. If your calipers are newer/cleaner then it might not be an issue, other then that the paint goes on easy, but it can run quickly when trying to hit all the nooks and crannies so you may want to do the caliper and wait 5-10 minutes before trying to get the hard to reach spots.