I've used one for lacquer, I can't remember if I've used one for varnish. I remember lacquering up a burl clock or 2. Works just fine and easier clean up and way less thinner to get it clean. The air brush was a good one made by Binks, I don't know if that had a bearing on the outcome. It wouldn't work on water based I don't think. The vintage on this air brush was before they became common and the tip sizes with mine would be too small. I don't know if the new ones will.
I also have a touch up gun, $30C here at one of our favorite chain stores, Princess Auto (Canadian Harbor Freight I think). Chinese made so available under many names in many places. Actually does a decent job.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.