Apparently, you don't, really. The rust I mentioned comes from
letting the car sit parked outside overnight where condensation (dew)
forms on the rotors, forms drips on the inside and outside edges,
rusts in those areas, and is insufficiently cleared the following day
because the driver's normal braking pattern is too tentative or
gentle. The rust then slowly starts cutting away the surface of the
pad (it's harder than the pad) and the pad gets progressively worse at
clearing the rust which obviously continues cutting away more pad.
Quite a nasty syndrome that's easily addressed by braking more
aggressively to be sure to knock off all the rust daily. Driving
through water has almost nothing to do with it. The greatest problems
with rust on rotors come from the car *sitting*, not being driven.
For an individual driver, that's correct, but some drivers can still
extend their pads' lives by 'smart braking', so your mileage *still*
Before I start, let me just say that I usually replace the rotors each
time I replace the pads. It's easier, gives me peace of mind, they look
pretty, etc.... 8-)
As far as cutting a rotor on a lathe is concerned, it really doesn't
impact the overall ability of the part to dissipate heat by a meaningful
amount as long as it is still within spec. Consider that a normal
"turning" results in only a few thousandths of an inch of material being
removed from each side of the rotor. Visit your local machine shop and
see how much powdered metal is shaved off a rotor being cut. I suspect
that if you weighed the rotor before and after the difference would be
insignificant. So you're really not changing the ability of the rotor
to act as a heat sink if you're not taking away a meaningful amount of
mass and other factors remain constant.
But like I said, I always just put new ones on my car. It's easier.
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