I admit that I don’t varnish a lot of my paintings. Many of them have white backgrounds, and I really want that background to be as bold as the surface allows it to be. Varnish is great for paintings with color, or at least that’s my experience. Varnishing is definitely good for protecting a painting over time. One needs to choose the correct varnish, though. You don’t want to buy a varnish that yellows over time. I fully recommend checking out his articles.
Click on the link above to read the full article on how to varnish a painting.