A trinagular file is good for dinged threads mid length. A very fine knife edgefile isbetter as it allows a finer cut to the coect angle.

A flat flile held near the 30 degree angele and cutting from the OD to the center will give you the best cleanup of the threads on the end of a screw. If I cut off a screw, I grind the end to the 30 degree angle, grinding so the flashing pulls to the center. Touch up with the triangle or knife edge file and you have a professional finished end on the screw. It gives you the same end profile that hitting the end of the shaft before threading it with the thread cutting bit on the lathe gives you. The file does a rougher, but acceptable version of this chamfer on the end of the screw.

Don