Ive never cut a bike fender but I am a fabricator by trade.The cut off wheel is really the only way,just take it easy and have some one help by squirting some water on the wheel while doing it.This will keep the heat down and avoid scorching the paint.One trick I have seen done is to use duct tape on both sides of the line you wish to cut on,this can save from a few minor walk offs when first starting your cut on the line.Another trick is to use a less then full diameter wheel as it will be stiffer and less likely to walk.
