We use a lot of spar varnish. But it's rare to use an entire quart within 2 or 3 days. And after about the 3rd or 4th day, when we reopen a can we find that it's started to gel. And once it's started, there's no stopping it. Time to toss it out.
How do you keep your varnish from spoiling? Ideally, it would be sold in plastic airtight pouches with a resealable nipple or pour spout. You could pour out what you need and recap the container without ever letting air into the container.
Until that time, what's your solution??