Q: I build, restore and repair wood and canvas canoes and use spar varnish on all exposed wood surfaces. I also use varnish on some indoor jobs: for example, I have always liked the gentle warming of color that varnish gives to pine paneling. I have noted that varnish cans, though appearing to be no different than other paint cans, are often difficult to close securely. Are the cans really different? Related to that question, varnish always forms a skin when a can is left on the shelf for some time, even if the can is sealed securely. What causes that skin and can it be prevented from forming, thereby giving the varnish longer shelf life?
A: No, the cans are not any different. As for the skin, it is caused by the same thing that causes the finish to cure: oxygen. In this case, it comes from the air in the space above the finish, called the “headroom” in finishing parlance.
Oil varnish is an oxygen polymerizing finish. That means it takes oxygen from the air and uses it to crosslink the finish molecules, turning them from a liquid into a solid. The skin is simply cured finish, and there are many ways to avoid it.
One way is to repackage the remaining finish into a smaller container where the finish fills it to the brim, thus eliminating any air. You can do the same thing by making the finish fit the container: add marbles to the liquid until it fills to the brim of whatever can it is in. Another is to add an air barrier on the finish — something that prevents the air in the head room from getting to the varnish. A piece of Saran™ Wrap pressed onto the surface of the liquid will do nicely.
The third common way is to remove the oxygen from the can. This sounds difficult, but is actually very simple. There is a product called BlOxygen, which you will find at Rockler and other woodworking specialty stores, that consists of pressurized deoxygenated air in an aerosol can. Spray the air into the can before affixing the lid, and it displaces the oxygen-laden air and replaces it with a combination of gases that contains no oxygen. That means no skin. The stuff works great; I have several cans in my shop and find it incredibly handy. By the way, I’ve also used it to keep guacamole from turning brown in the fridge, provided the container is air-tight.