I use Gamvar varnish and a Liquitex Freesyle brush, 4 inch.
Prior to Gamvar I do one very thin coat of Galkyd, thinned a bit with OMS. This acts as a first coat of varnish but but is actually a medium and will bond with the paint layers of the painting.
It can still be painted back into if you see something that needs work and allows you to continue to modify the painting. It evens out the sunk in (dull) areas of the surface so that it will take the varnish nicely.
Here's a video I made of the Gamvar varnish application.