Join Date: Jul 2016
Location: Seattle, WA, USA
The Rear Rotor being about .200" (I'm talking inches here) you could loose .050" and still be safe. After that you are going to start getting Hot Fast and warping.
Rear Rotors are not that expensive. It's going to cost you enough to get it resurfaced that you might as well get a new one.
As your rotor wears and you replace the pads, you can take a large flat medium grit sharpening stone and hand dress both sides. Use a Tangent direction and then a reverse Tangent to give a nice Cross-hatch pattern. Stop each trip around the disk where you began. You are not trying to completely renew the original surface; you are taking the high spots down that are otherwise going to eat up your new pads. Clean the grit off and install the new pads.
The flat sharpening stone with the work done by hand will keep the disk flat Don't use power tools of any kind to do this because there is no way to keep it uniform and you will develop pulsing and chattering brakes. I don't run cheap pads so it's worth my while to do this work and it will keep your disk flat thru it's useful life .
Something to keep in mind with rear brakes. Many people think they are not using their rear brake because they don't very often step on the rear peddle. The fact is, your ABS is apportioning 30% to 40% to the rear brake when using the hand lever; even on a Bare Bones none Servo Assist version. My '08 1200 S is all bare bones (no ride control either) and I wore out my rear pads on 10,000 mile trip doing the USA Loop.
While I'm on the subject, most people don't flush their brake fluid in the entire time they own their bike. Then they piss themselves when the ABS fucks up and it costs them over $1000 to fix it. If you haven't flushed the system in a couple years or maybe ever, what comes out will scare you. It's black and maybe even with lumps floating around. Keep fresh fluid in your system and it will prolong the live of your expensive ABS unit.