First, what is rosin? Rosin is made from pine pitch that has been oiled and has become hard, like glass. It makes the horsehair on your bow tacky so that when you pull the bow across the strings it grabs the string slightly as it goes, which makes the string vibrate. Without rosin on your bow hairs, your bow will barely create a sound when pulled across your strings.
How to Rosin Your Bow the First Time
When you get a new bow, or get your bow back from being rehaired, it will usually not have any rosin on it. Before you can play with it, you will need to prime the hairs with rosin.
There are 2 types of rosin: one comes in a rectangular piece of wood, and the others are cylindrical cakes glued to a soft cloth. We prefer the ones glued to the cloth because you can swipe the rosin across in all directions, thereby using more of the rosin. The ones in wood tend to get a groove right down the middle and a good portion of the rosin never gets used.
Before rosining your bow, tighten the nut at the end of the bow clockwise until you can just get your pinky finger between the hair and the stick in the center of the bow. Be careful not to over tighten your bow. You do not want the bow stick to be straight or curved up and away from the hair. The bow should still have a slight curve to it, with the curve down towards the hair.
Hold the bow with the proper playing grip, except position your middle finger slightly in front of the metal ferrule to keep it from jabbing into your cake of rosin and breaking off little chunks of your rosin.
Hold the cake of rosin so that your thumb and first finger stick up along the edges of the cake. This will help keep the hair from slipping off the edge of the rosin.
Start applying the rosin to your bow hair in short, firm strokes in sections of 5-6". You can either move your bow arm, sliding the bow over the rosin, or you can hold the bow still and slide the rosin over it. It is a matter of personal preference, so try both ways to see which you like better. Sometimes it is hard to get the rosin going and you may have to rough it up a bit first with a bit of sandpaper or the sharp, front edge of your ferrule. You will know when the rosin starts going because the cake of rosin will not be shiny anymore.
The first time you rosin your bow, it will take quite a bit of rosin. Cover the entire length of the hair, going over each section several times. When you are done with each section, do a couple long strokes up and down the entire bow to help distribute the rosin evenly over the surface of the hairs along the entire length. Put a few extra strokes in the area close to the frog. (the part you hold)
The way to tell if you have enough rosin on the bow is to simply take your instrument and start playing. You should get a good bite on the strings, and the instrument should make the proper sound. If the bow slides around, you need more rosin. You can also look at the bow and see slick spots on the hair. These spots need more rosin. If a lot of rosin dust flies around while you play, you have too much rosin on your bow. If the bow feels slippery on the strings, then you need more rosin.
You can get too much rosin on your bow. If this happens, your sound will be fuzzy. You will hear a lot of surface noise when you play and you will see rosin powder all over your instrument. You can remove excess rosin simply by wiping the hair with a clean soft dry cloth along its length.
Your bow may not need to be rosined before every practice. Usually every couple practices is frequent enough. Once your bow is rosined for the first time, a few complete strokes of rosin, with a little extra close to the frog, should be plenty.
After every practice session, you will want to loosen the hair on the bow to protect the bow. You should also wipe down both the bow stick and your instrument with a soft cloth to protect the varnish from the rosin dust.
A note about rosin: Dark colored rosin is good for dryer climates and creates a little stickier bow when there is not much moisture in the air, while lighter colored rosins are good for more humid climates. So, if you travel, or live in an area with micro-climates, you will want to have one of each in your case.