Rehair, replacing grip/leather, straightening, recambering, broken bone tip, cracked frog, stripped or loose eyelet, broken mother-of-pearl slide, cracked stick at mortise or head, missing pearl dots, loose or missing heelplate, broken stick.
A bow should be rehaired when enough hair has broken so that the bow lacks a complete width of hair at the frog, or else seasonally, when weather/humidity changes make the hair too tight or too loose (every 3-6 months).
There is no benefit in using an excessive amount of hair, which just dampens the sound. The actual number of hairs varies with the quality of the hair (better hair is thicker). A special gauge to measure the correct amount of hair for stringed instrument bows is used.
Horses from colder climates with limited food supplies traditionally produce the best hair for making bow hair hanks. Hair from these horses grow relatively slowly, allowing it to develop a much finer structure. In addition, low rates of infection and illness make it possible for the hair to grow very evenly. The best quality hair originates in Mongolia and Siberia. Bleached and (or) stretched hair should be avoided because it breaks easily.
The most important difference is density. Lightweight, generally brown mass-produced brazilwood bows range in price from $80NZ to $500NZ. Some are difficult to rehair and repair because the mortises in frogs and bow tips often are rapidly and crudely cut; consequently wedges tend to pop out. Also, brazilwood bows need frequent straightening and recambering — the wood is not dense enough to hold a proper curve. These bows are affordable and perform adequately, but expect problems!
Pernambuco is the denser, stiffer grade of Brazilian wood, which is used in handmade professional-quality bows that range in price from $400NZ to $50,000NZ. It is identified by brown to orange-red colour, closeness of wood grain, and high density (most sticks sink in water). It stays straight and holds its camber very well. The overall craftsmanship, weight, and balance of these bows usually are excellent. Consequently, they play well, are easy to rehair, and are subject to few problems. Pernambuco is the obvious choice for serious players.