In various threads around the forum there are mentions of faulty rear discs and their replacement. I am unclear as to what the problem with them is. In my case the front discs are smooth and polished from the outer edge to virtually the centre. The rears however are scored, not well polished with a noticeable rust band on the outer and inner edge where the pads don't reach. It seems the rear brakes are very lightly used. Is this others experience?