To Varnish or Not to Varnish?
This is a question I receive as much as any other question. I do NOT varnish my paintings. When I first began painting acrylic abstracts on canvas I did varnish all of the paintings because that is what I heard you should do. I experimented with all of the various finishes and was not happy with any of them. For me they make the surface of my paintings too homogeneous.
What I have been doing instead for the last couple of years provides more contrast to the surface of my work which I now prefer. when I have dark values near my centers of interest I like to add gloss gel medium to those areas to make them rich and glossy. These areas also receive thicker paint and I like the contrast provided by the thick glossy areas and the matte finish of the rest of the painting. The contrast in surface finish to me is more important than providing a protective finish over the entire painting.
Like so many other decisions in painting it's a personal decision. You have to do what works best for you. Right now for me not using varnish is what works for me. Do what works for you.