Bob LaCivita replies: When sharpening a back saw you must first determine the number of teeth per inch or points per inch (PPI). When sharpening my dovetail saws which range from 15-21 PPI, I use a 4˝ double extra slim triangular file. When sharpening a 9-10 point saw I use a 6˝ extra slim file.
A simple way to determine this is to set the file in the gullet and visually see if the file fits. If it fits it should work.
I am talking very fine mill files here. The question asked about grooved files and I do not know what that means. Remember finding the file is the easy part. When filing the saw use a saw vice or saw chops. File every tooth pointing toward you with the same number of strokes keeping the pressure even. Turn the saw around and repeat. The teeth should look even and the gullets should be the same size. Try the saw.
If it pulls left or right you could have a few problems.