Spotting hairs is fun!!!!!
Ok so I lie, its a pain in the ass.
Use a good sable brush *Windsor Newton #3 is what I use*
I do not like nose hair brushes # 1 or smaller, that a lot of people use , The spot tone should be brought into the brush and then dropped onto the print with an amazing point , only a # 2 or # 3 can give you that kind of fine point and the ability to drop the ink on paper. If you do not understand this , think of a holding tank of ink that is connected to a very sharp point that you then use to drop drops of ink to selected light areas of a print.
As Bill says practice before you go to the live print.
Remember that you need to * charge* your brush with spot tone that matches the tone that surrounds the line... It needs to be spotted , not dragged, into the line. I like Nicholas idea of breaking it up . The idea in spotting is camofllage and you are not trying to hide the line in one single sweep but a series of light spots that join together and eventually ****visually ***** hides the line.
I have spotted thousands of prints and I trust very few people to spot for me , it takes patience and skill that only can come from practice.
Building up the tones is the trick and you will find after awhile it very easy to do.
have fun