From "Green Valley Boat Works - Builder's Notes: The Varnishing Point", at http://www.greenval.com/FAQvarnishing.html

"...clean off the rim of the varnish can, close the lid firmly, and turn the can upside down. It should rest with one edge up on something about 1/4 to 1/2 inch high to keep the can tipped. Turning it upside down helps seal out any air leaks which can prematurely dry up the varnish, and tipping it stops the formation of 'stalactites' from the bottom of the can."

I used this technique with great success with my Z-Spar Captain's. That is, until there was so little left in the can that it would no longer seal the rim when upside down.

Chris

: I have the same experience when a can gets below half full. My theory is that
: when the less than half filled can is closed there is enough oxygen inside
: the headspace to gel the remaining varnish. By this point, the can has
: also seen several air changes from repeated openings and the viscosity is
: already on the rise. It's a classic case of administering a coup-de-gras.
: One option is to use bloxygen or propane everytime the can is opened, or
: keep transfering the varnish to smaller containers as it is used.