rub (something, especially a violin bow or string) with rosin

island musicians are rosining their bows

resin, especially the solid amber residue obtained after the distillation of crude turpentine oleoresin, or of naphtha extract from pine stumps. It is used in adhesives, varnishes, and inks and for treating the bows of stringed instruments

they will also supply crushed colophony rosin

liquid rosin flux