I'm in the process of refinishing a bunch of windows and doors. I'm in and out of a can of spar varnish every day. No matter how tightly I seal the can, by the time I'm halfway down, it starts to gel. I've tried the Bloxygen route and had no success.
When my kids were younger they were bottle fed. The plastic bottles had a plastic bag liner that collapsed as the kids drank. That way they never drank air. Why can't the varnish makers come out with similar air tight packaging?
Grumble, grumble, grumble......... Damn kids get off my lawn!