That's the nature of varnish. It's purpose for the last several hundred years, or so, is to act as a barrier between the grime and the paint. It's easier to remove dirt and nicotine from varnish then from paint. A properly applied coat of varnish can be removed usually with little problem. It is still a tedious and specialized process and should be performed by someone who knows what their doing.
Varnish also brings out the dark areas which have gone flat after drying.
Never use linseed oil to coat your final painting. In time it turns dark brown and hardens and is much more difficult to remove then varnish. Having hands on contact with so many different paintings and many different styles and techniques has shown me what worked and what doesn't. Using a magnifying headset gives one a finite view into the valleys of paint that even the artist never saw. It's changed the way I paint for the better.