brake parts, pay sooner or more later....
New rotors are Blanchard ground to provide exact thickness, parallelism, flatness & that non directional finish due to the orbiting motion of the grinding wheels. Rotor wear results in a taper & non-flatness on the working surfaces, sanding will make the surfaces look better & flatter, but the taper & out of parallelism will still exist. Excessive taper can damage the caliper,if the pads are run too long, due to uneven pressure where the piston & caliper squeezes on the pads,& the piston(s) are allowed to protrude out of the caliper too far. New pads on an out of spec or otherwise damaged rotor will make noise until the pads wear in to the rotor, resulting in a less than ideal condition that will wear on the whole assembly, given time in service.
( bushings & slide pins included) Sanding on a new rotor is unnecessary & a waste of effort & time. :ohmy:
As previously mentioned, cleanliness is of importance.