1 …put more rosin on?
Short answer: when you’re not getting the sound you expect
A fairly good rule of thumb is to add rosin for ever hour or so of actual playing time. Don’t put too much on, though, just a bit of a rub at each end with maybe two passes, down and up, along the length of the hairs, more if you haven’t done it for a while. If you see a cloud of white puff up as you play, you’ve put way too much on! Whip your bow through the air a few times to get rid of the excess, but try not to bang it into anything or anybody as you do!
2 …rehair my bow?
Short answer: when the hairs slip or don’t make sound the way they usually do
Bow hairs are real hairs taken from the tail of a horse, so they will stretch, break, and especially with sticky rosin on them, pick up dust and dirt. If when you grasp your bow you regularly also grab the bow hairs, your sweat and finger oils will resist the rosin, like oil and water, and create a slick spot on your bow that won’t take rosin. That’s okay at the frog but if it’s farther up, you’ll have a problem getting sound at that spot. The bow will slide by. Now that’s a bad feeling!
A slick spot can also be caused if the rosin picks up dirt or oil for other reasons. The safest place for your bow is in your case or on a music stand. If you leave the bow lying where it can pick up dirt and dust from a floor or other area, you may end up with a spot that won’t play very well. It’s not a good idea to try to clean the hairs yourself. There’s a risk you might ruin the finish of the wooden stick, and it may weaken the hairs so that they don’t play as well.
The main reason I rehair my bow is when enough hairs break to affect the sound or threaten the bow. Most players slightly tilt their bow, so one side of the bow hairs gets more contact than the other and is more likely to break hairs eventually. There are about 240 hairs on the bow, so breaking a few is not a big deal, but if you get enough hairs breaking from one side, it can apply pressure that slightly twists your bow. If your bow is weak or you don’t tighten it enough, the breaking could even happen in the center of the bow hairs because the wood of the stick is closest there and can hit the strings more often, possibly straining or breaking the hairs.
Another reason for rehairing is if the hairs have stretched out enough that you can’t tighten the bow. This is only likely to happen if you have a habit of not loosening the hairs when you’re done playing. Depending on the weather, bow hairs, like human hairs, will stretch and contract. If you leave your bow tight it will stretch the hairs, and during the summer, when warm humid air stretches the hairs even more, the hairs could stretch to the point where the bowscrew reaches the end of its range and can’t tighten your bow any more for playing. You’ll have to get the bow rehaired then, and try to remember to loosen the hairs when you’re done playing!
3 … look for a new bow?
Short answer: Any time — it’s fun!
It takes a while to tune your ears into how new bows sound. The ideal situation is if you can take a bow, or several, home to try over the course of a few days (many shops allow this, and Shar Music offers a mailing service with 4 bows at a time), so that you can get the feel and sound of your own bow out of your mind and really appreciate the new bows. Try playing the same short sequence of music on each bow in order to compare. The weight, balance, and sound of each bow will be different, but you might not notice the difference immediately. Sometimes a bow really makes you sound better and more comfortable. Trying new bows could even make you appreciate what you already have! Price is not always an indicator of which bow is best for you and for your violin. You have to listen carefully, and even ask others to listen too. If you’re thinking of trying out new violins soon, get the violin before you try out new bows.