This page, with accompanying slideshows and photos, is the most comprehensive article on how to varnish a painting I’ve ever seen. It’s put together by NitpickyArtist.com, which has a lot of good technical info including advice on how to reduce glare, how to enter juried shows, or even info about table surfaces and easels. You may not want to follow this person’s advice all the way through, as it’s quite complex, but there are a ton of good ideas in here to use otherwise. It was the section at the bottom on how to clean a varnish brush that caught my interest initially.
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.