People often ask about how and when to varnish a painting and many methods are used. A lot prefer the tried and trusted Damar Varnish method and Liquin is often suggested by even very well respected artists. There are many methods but the video below is from one of the most respected medium and materials suppliers in the art world.
With full respect to all the other methods used in varnishing a painting this little video covers many of the topics an artist should consider when applying a varnish layer.
Gamblin Picture Varnishes from Gamblin Artists Colors on Vimeo.
For more information on Gamblin and their products, you can explore their site.