10/02/2023
Under a microscope, the hairs of a bow look like the teeth of a saw. To keep these teeth sharp when putting on rosin, it is best only to rub in one direction. Use a series of long down bow strokes, beginning at the heel. Protect the rosin from chipping by putting your middle finger over the ferrule (the metal band on the frog from where the hair emerges).
The exception is when putting rosin for the first time on new hair. Then it is best to use short back and forth strokes of about six inches. Begin at the heel and gradually move up the hair. Judge the speed you move up by the feel of the hair on the rosin. As long as the hair feels rough, there is not enough rosin: when there is sufficient, the hair feels smoother. Go along the length of the bow twice, followed by between eight and twenty long down bow strokes, according to the feel of the hair on the rosin, i.e. rough or smooth. (This process is not necessary if your bow rehairer has put rosin on for you.)
This approach can also be used on a bow with hair that has not been rosined for some time, or is old. Obviously, less rosin will be needed than with new hair.