rtparsjr you found a nice video about varnishing, and this is your solution. I do not know about retouch varnish use like Andrew does, my theoretical knowledge says to use it as a bonding medium to apply new layer of paint in the top. I would reiterate those 5 points like in the video.
I would add that I liked using spray varnish quite a bit because of the nice shimmer it was giving. Now I have only W&N gloss and matte varnishes for brush application and I mix them to a bit glossier side.