I have just opened a quarter full tin of varnish that had not been used for about two years. It was perfect inside and had no skin. I then remembered that when I last used it I had just been given the tip of filling the tin with gas before putting the lid on. Try it, it really works. All you need is a gas blowtorch to spray some gas into the tin of varnish or paint and chase out all the oxygen before putting the lid on. Don't light the torch of course!
Mike