: As a can gets used, air in the can thickens the varnish.

I remember a trick I heard of in a woodworking magazine years ago. As your can of varnish has less varnish and more air in it, the air starts to oxidize the varnish and that forms a the skin on the top. A way of slowing this process down is to exhale into the can right before you put the lid on. Supposedly, carbon dioxide is better to have in the can than oxygen. I have done this for years and I think I have better luck making my left over finishes last longer. I usually hold by breath for a while before exhaling.

I know it sounds a little wierd and I have not done a side by side test, but what the heck, I'll share it anyway

Steve