Pirastro Piranito Rosin for Violin

High quality rosin allows for perfect bowing without scratchy sounds, especially if only a thin layer of rosin is applied. The general rule is:
violinists and violists use a harder, dryer rosin
cellists use a medium strong rosin
bassists a softer rosin

Another important aspect for choosing the right rosin is the core material of the string that are being used:
steel strings are easier played if you use a hard, dry rosin
for synthetic, gut and gut wound strings use a softer rosin