The fundamentals of varnishing are the same as for a painting without much surface texture, except for two things. You should wait longer before varnishing to be sure that all the thick paint is completely dry. If it's an oil painting you need to get to an exhibition or have sold, apply retouch varnish if you're in any doubt about the paint being dry.
Secondly, as well as brushing across the canvas in a sweeping motion, use the brush in a gentle stabbing action in the highly textured areas. You want to ensure the varnish gets into every nook and cranny, without puddling and without creating air bubbles or froth. Work systematically across the painting, and avoid going over areas where the varnish has started to dry as you risk making it cloudy.
