I haven’t tried the stop loss kit but that seems like it would do the job. Basically you have to try to keep the varnish oxygen free when not using it. I read a bunch of other techniques online like blowing into the can before closing it, using can of compressed air/duster, using gallon freezer bags, etc. The stop loss kit looks like a professional (and less messy) version of freezer bags. Maybe if I have to order another can of varnish I’ll pickup the kit and give it a try.
-mike