And if I do that, what can I do to keep that surplus varnish from hardening, so that I might use it at the end of the paddling season to touch up the finish? Thanks for any advice you can offer.

Hi William,
As Brian posted, use an inert gas like Bloxygen or similar. Or get a couple of bags of children's glass marbles and pour them in as you empty the can, That minimises the space left at the top.
One thing about glass marbles, they're immenently reusable.

Mike Savage
South West Cork