I don't use a varnish, I find the resin cures hard enough to not need it for normal use. If you're using something softer like epoxy you'll probably want something but I don't have any experience there.

If you don't mind me asking what kind of resin are you using?

I use urushi and sometimes cashew lacquer. These may not truly be "resin" if that word has a specific technical meaning. Some of my original sources used resin as a translation for the urushi material so I do too.

Thanks, for the info. I appreciate your time.