I buy varnish by the gallon because it’s more economical, but after a few uses, the rim fills with gunk and the finish begins to skin over inside the can. As a result, I usually have to throw the can away before it’s empty.
I solved this problem by recycling a 5-liter wine box into an airtight varnish dispenser. Choose a box with a twist-open spout. After enjoying the wine (and a good night’s sleep!), remove the empty bag and carefully pry the spout out of its fitting. Rinse the bag and let it dry.
Filling the bag with varnish is a two-person job: One person holds a funnel and the bag while the other person pours. Stir the varnish thoroughly before pouring it. After pouring, remove any air that remains inside the bag by carefully compressing it on a table, fitting side up. While the bag is compressed, open the spout and press it back into the fitting. Make sure the spout is oriented to operate correctly when the bag is back inside the box.
Because the bag is air-free, once it’s reinstalled, you’ll be able to stir the varnish by simply shaking the box. A classy label ensures that no one mistakes your polyurethane for pinot noir.