grinding disks is only a way to destroy yoru rotor and pads...... if you want better breaking with a disk buy a good brake i've had it explained to me many times over the years why its useless, fact is (as mentioned) both surfaces need to be smooth, if all else fails, clean yoru rotor with a baby wipe every few week, and leave it for about 5 mins for it to evaporate off (works same on rim brakes, but lasts about 15 mins)