Above is a photo of one of my oil sketches. To the left of the line, the painting is varnished, to the right it is not. Why varnish a painting and also, how, where and when?
Picture varnish acts as a transparent layer and gives greater intensity to the colours it covers, a bit like rain water on a stone (compare in the painting above the colour of the hut on the left which is varnished and the unvarnished hut on the right). Varnish also protects the painting from stains and moisture.
When painting my first portrait at art school I remember being struck, after just an hour or so, by how matt and dull it looked. This was “the paint sinking into the canvas”. The solution to retrieving the original fresh colour is to cover the paint with a retouch varnish which is usually a mixture of varnish and turpentine. It needs some deftness as the paint is usually still wet and the application could spread the paint, ruining carefully placed touches and flourishes of colour. Once a painting is finished and dry, I apply a full coat of picture varnish.
I try to make sure a painting is thoroughly dry before varnishing it. Waiting for a painting to dry can take months especially if I have been using slow drying paints like Ivory Black. If the paintings contains no black then I would consider varnishing pictures after a month or so of drying, but then that is because I paint thinly.
It is best to varnish paintings on a warm day so that they can dry quickly, and in a dust-free place away from draughts so that they look fresh and clean and not like someone has just emptied a vacuum cleaner on them. I keep the paintings flat during drying to stop the varnish from dripping down in ugly streaks.