Avoid a build-up of rosin by dusting off after each session. Periodic cleaning with the right substance is also recommended. One popular cleaning fluid is Hidersol Varnish Restorer, but remember to apply this product to a small area of varnish initially in case of an adverse reaction. The best way to apply cleaning fluid to your instrument is as if polishing shoes. A tiny amount smeared on the corner of a dry soft cloth is all that is needed. Rub gently to dislodge the rosin or dirt and then buff it off with a dry part of the cloth. That’s all there is to it!