did the old masters varnish their works? what problems are there if you do not varnish?
I have found that the varnish gives my paintings an even sheen over the entire painting. When I paint I do not always mix the medium evenly with the paint on the brush and when applying it to the canvas, therefore the varnish evens it all out.
The old masters did not varnish very often as you can see in museums the painting have cracks where they have not been properly preserved and have dried out unevenly. Fading of the pigment is also a sign of an improperly preserved painting.
I think it wise to varnish at least with a lite coat of satin finish varnish about 6 months after you finish work on your paintings to preserve your work long after you are gone, you never know who might be admiring your work. Just my thoughts.