some bugger put a sticker on my cello, so i've tried to wash it off with a weak hand soap + water solution, but there's still a sticky residue. what do?
Anything designed to get sticky stuff off should work. As for what won’t damage the varnish, I’d say go for rosin cleaner or a VERY small amount of Goo Gone on a folded paper towel. Rub in small circles. If the varnish starts looking cloudy as you’re rubbing, stop and switch to a dry section of paper towel.
And if all else fails, I find spit and a finger to be particularly useful at getting what water won’t. Disgusting? Sure, but don’t knock it til you try it.