I've spoken to Gougeon technical support when I was trying to decide which hardener to use, and they actually disrecommended 205 and 206 hardener for clear-coating applications. They said that over time, epoxy with those hardeners can cloud-up; especially if not throroughly mixed. I posted a fairly lengthy report of my conversion with them in an older post. You may be able to find it in the archive.

I used 207 and haven't seen any problems. For varnish, I used Z-Spar Flagship.