The term ‘coarseness’ refers to how rough or smooth the face of a rasp is. This is based on the size of the teeth and the distance between them.
The coarseness of a rasp can be measured in three different ways.
American pattern system
Some rasps will be listed as coarse, bastard, second cut, smooth or dead smooth in a similar way to files.
Teeth per row
Other rasps may be described as having a certain number of teeth per row. This refers to the number of times the triangular punch has been hit during each diagonal pass across the face of the tool.
The higher the number of teeth per row, the finer the rasp will be.
Grades
Rasps can also be measured in grades, from 1-15, with 1 being the coarsest and 15 being the finest. Grade 1 has a small number of large teeth on each row, whereas grade 15 has a lot of very small teeth.
Grade 1 is so coarse that it tends to only be produced for industrial use!
Whichever system is being used, all you need to do is choose a grade, cut or number of teeth per row that will be fine enough for the finish you would like.