After my unscheduled get-off at MOOTMag 7, my front brake would cause the lever to pulsate at low speeds. Allen tilted the bike up on the back wheel and sidestand, and Greg spun the front wheel -- it stuck, and wouldn't turn completely around. Rotor warped, or bent, or whatever. So I added rotor to the parts needed list. Received one from a fellow MOOTster a few days later.
So I decided to check out the replacement rotor, and was surprised to discover it was warped, too. Not as bad as mine, and in a more complex pattern than mine, but warped nonetheless. Since I had mounted the tire/wheel on the axle in my vice to polish the wheel, it was a simple matter to rig up the "feeler" shown in the first picture to check out the rotor flatness. Just a piece of copper wire about AWG 7 to 9 or thereabouts -- I had in my electrical junk box.
With a light behind the setup, one can use the reflection of the end of the wire from the rotor surface to obtain a very sensitive indication of warp when one spins the tire/wheel. Notice the "soft jaws" addition to the vice, preventing marring the axle (thanks to MagnaDaddy of Hutto for showing me this cool addition to a vice -- magnets hold the jaws in place to the vice jaws.)
Brought it back to planar using a soft face (brass) hammer. I practiced on one rotor before tackling the other. Go slow, it takes some time. "Sneak up on it" by whacking gently, measure, whack a little harder, measure, etc. until it yields just a bit. My rotor was mostly planar except for one spot that had to be whacked from the bottom (through the wheel), which I did by using the end of the hammer instead of the face. The replacement I received required a few whacks in both directions. So now I have two almost perfect rotors.
Caveat -- I have not tried either by riding the bike. Stationary tests, just spinning up the wheel, show a huge improvement from where I started. When I spin the front wheel now, it stops at more or less random spots as it spins down against the slight drag of the front brake pads.
It is possible that the rotor will go back to its warped state once it gets a bit hot from stops. I will report more once I get the bike reassembled and ride it some. Straightening with hammer blows (instead of steady pressure, as in a humongous hydraulic press) is less likely to leave internal stresses in the metal, so I am hopeful this will be a permanent fix...