if its a fine scratch but deep, sometimes just plain old floquil primer will fill it after about 2 or 3 coats, then wet sand it. this has worked for me on ocassion.
another way i fill cracks sometimes is (and this is not a practice that has been posted around here anywhere but it has worked for me a time or 2) to sand up some plastic dust so that it fills the crack you are after, then, take some Tenax-7 and touch a little to the crack and it will "weld" the dust in place. do this a couple of times then sand with fine sand paper. i've got this to work on occasion. later.