Now this doesn't answer your question but...
I haven't used the flat varnish but I have experimented with the gloss and satin varnish on an old build. It seems to go on very well. I hand brushed and airbrushed it on and it gave very good results. I'm planning on using the gloss on one of my next builds.
So as far as applying it goes I can tell you it works pretty well. I don't know how the flat coat will look, though.