Its oil in the cut outs which yeah unless you immerse it then give it a good scrub e will stay there.
Could be leaking seals too.
Iso bath or maybe a degreaser bath? Or degreaser then iso? Or degreaser then meths then iso...
I've found another rotor so might just try that with new pads.
Whichever, give it a good scrub with a brush and rinse it clean.
@hippy started
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Its oil in the cut outs which yeah unless you immerse it then give it a good scrub e will stay there.
Could be leaking seals too.