It is important to choose the right varnish for your oil painting, if you're looking for a varnish to apply to your finished painting you should probably go for Dammar Varnish, or Picture Varnish for a longer lasting protection, note that these 2 mediums should only be apply to completely dry paintings.
What could happen if you varnished a touch dry painting (which was dry to the touch but not entirely dry) with Damar / Picture Varnish? When you apply varnish to a touch dry painting, it won't allow air through the varnish layer, this will stop the paint from drying properly, if the paint is half dry it woulnd't have a strong bond between layers. Also, the varnish layer may crack.
What exactly is a Retouching varnish?
Retouching varnish is used during the course of a painting, sometimes some areas of your painting lost its vitality and become dull this is due to what is known as "sinking", when the top layer of oil has 'sunk' to the layer underneath,and only pigment is left on the surface, retouching varnish could be applied as a temporary varnish to restore colors, that means retouching varnish can be applied to touch dry painting. You can paint over Retouching varnish.