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Hi,
If you have grooves in the pad and rotor surface, it is due to uneven wear from the pads. Some pads are semi-metalic, that is, they have bits of metal imbedded into the pad material. This makes the brakes more fade-resistant, but at the cost of increasing wear to the rotors.
These metal bits wear at a lesser rate than the surrounding pad material and are much harder. Consequently, they wear into the rotor at a greater rate than the surrounding non-metallic friction material. The result is grooved rotors as you describe. Also, sand and grit can also stick to hot friction material and cause the same thing, but to a lesser extent, so some grooving is almost always present when you change the pads.
Whenever you change pads, you should machine the rotors too. This will even them out and provide a uniform rough surface to allow the new pads to bed-in properly and evenly. If your rotors are grooved, you should have them machined to the point where they are even again before replacing the pads. Failure to do so can prevent the new pads from properly bedding in and lead to premature wear, pulsating pedal, uneven braking (pull to Lft. or Rgt.), etc.
The rotors on the Boxster have a fairly tight tolerance. They seem to last for only 2 sets of pads before requiring replacement. The wear limit on the rotors is 22.6mm (0.89")Fr., 18.6mm (0.71") Rr., less than this and they should be replaced. The rotors can be machined to 22.0mm(0.87") Fr. and 18.0mm(0.71") Rr. assuming they exceeded the above dimensions to begin with.
You might want to mic the rotors to insure that they are within spec. Your brakes are really the most important component on the car. And their maintenance is not something you want to do haphazzardly or incorrectly. Hope this helps...
Happy Motoring!... Jim'99
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