To reduce the amount of air ( and particularly oxygen) in your stock can of varnish:

floating a barrier on the surface of the unused varnish is a good idea, if you can get it to work. Here is a simlar idea that may be easier.

For a quart can get a Baggie quart or gallon sized plastic bag, or a similar sized bag of some other brand. You want the thin bags that need to be sealed with a twist-tie. These are usually called foodstorage bags -- not the 'sandwich' bags, or the zipper or zip lock bags.

Stick the bottom of the bag into the partially filled can of varnish, gently open the top of the bag and puff a few breaths into it. You want to try to inflate the bag like a balloon. as you do so, you force the outside of the bag against the liquid varnish and the sides of the can. This greatly reduces the volume of air that can come in contact with the varnish.

Fold the excess plastic bag over the outside of the can and press on the lid. The plastic is so thin that the lid should pinch it tightly, but stioll close completely. Any air in the can is now trapped inside the bag, while the varnish is on the other side of the bag.

When you go to reuse the varnish, you may find it easier to lift the lid as the plastic bag serves as a barrier to keep the drips of varnish in the ring at the top of the can from gluing on the lid.

After opening the lid, just grab the overhang from the plastic bag and lift the whole bag out, and discard it.

Hope this helps

PGJ