Files will leave different finishes on a piece of material, depending on their coarseness.
The opposite of coarseness is fineness. A fine file has thinner teeth that are close together. A coarse file has thicker teeth that are further apart.
The finer the teeth cut into the file, the smoother the finish you can achieve. Using the smoothest files, you can create a mirror finish on steel.
This process may take longer but it’s worth persevering!
The coarser the teeth cut into the file, the quicker it will remove material from your workpiece, but the rougher the finish it will leave.
Single cut files also produce a smoother finish than double cut files. This is because their teeth are straight rather than diamond shaped, allowing for a more consistent cut.
Using a single cut, grade 6 Swiss pattern file will produce the smoothest available finish, whereas using a coarse, double cut American pattern file will produce the roughest finish.