You want a file to rough out body filler? Sorry, but bad idea. All of my bodywork skills come from knowledge handed down to me and taught, each with over 30 years experience.
sorry, I should have elaborated.. I dont do bodywork. I do fiberglass speaker boxes/amp racks/dash panels etc. finish work isnt as important as typical body work.. I may set up a panel with some duraglass and knock it down real quick when it starts to flash. the panel would typically get covered in carpet/vinyl/suede so minor imperfections arent as important.
in cases where the piece would get painted, the file would still be just to knock down an initial layer of duraglass, then followed up with some rage gold etc... I have multiple DA's that take over from there before some high build primer. at that point, its off to the bodyshop to seal then paint.
other than that, I may need to precision file an abs plastic dash kit that I molded. the files that nissan posted should handle that. ( I already have needle files but am always looking to purchase more tools. can never have too many, especiallya consumable item like these).
I would get a fairly coarse file (but definitely not a vixen) half round for the duraglass, and I wouldnt sand the duraglass with a file until it is fully hard, or the file will clog very quickly.
The above instructions I posted is for body panels, but I understand what you are doing.
Have you looked into the little 3" blue point DA? I havnt tried a file on duraglass, I use duraglass rarely for some areas, and rough it out with a DA once hard, and then follow with rage gold, or the new evercoat quantum filler. I think Duraglass will clog most any file very quickly.
Also look into evercoat slick-n-sand or sikkens Poly Surfacer. Both are polyester "sprayable body filler" products. They are polyester based, and spray on very thick, and then sand better than just about anything. The slick n sand doesnt build as much or sand as nicely as the polysurfacer, but the slick n sand is $70 a gallon, Poly Surfacer is $60 a quart. Both of these would be a great topcoat for rage gold, as they have great fill, and sand very easily. They take a lot of the final blocking work out of sanding filler.