Z-Spar says:

Z-Spar Captain's Varnish: A high solids marine varnish that is a polymerization of several different resins and drying oils. 1015 Captain's Varnish is recommended for the highest quality work. It has outstanding gloss retention and all around durability. It is considered by its many users to be the most durable varnish ever offered the boating trade. Contains Ultra-Violet Shield.

Z-Spar Flagship Varnish: 2015 Flagship Varnish offers the highest U.V. protection available today in a marine varnish. Gloss retention in Flagship is superior to the leading competition as shown in recent laboratory weatherometer testing. 2015 Flagship Varnish brushes, levels and sands with the same outstanding characteristics as 1015 Captain's Varnish. It can be built up to many coats for a deep, rich, mirrorlike finish. The depth of gloss is increased with each coat.

I don't see much difference in these two statements except for the gloss retention statement. I don't know how they achieve that chemically. We tried Flagship at the boat yard I work for and the guys who varnish here 52 weeks a year say they can't tell much different. They say Flagship dries faster which makes it harder to brush well. Flagship probably lasts longer, but I bet the difference is marginal. I think you made a good choice in using it. I use Captain's, but a lot of this is what you are used to.

The one phenolic based varnish does really outlast the others is Petite Bak-View, but it dries very fast and brushing results are only fair. It does look good sprayed and tests by Yachting Magazine 3-4 years ago put it 2-3 months ahead of everything else in an eight month durability test in bright Florida sunshine, including clear two part urethanes like Imron & Awlgrip. We have used it on clear finished cold molded wood hull sides and cabin trunks that have epoxy base coats and it has worked very well. It is too much trouble for regular brightwork.

As far as compatibility between Varnish & Epoxy, I think as long as the epoxy is well cured and sanded, it is pretty inert. The solvents in varnish are pretty mild and are unlikely to cause a problem, I've never seen any. If the varnish does not have strong UV resistance however, you can get degradation at the interface over a 4-5 year period that will result in the varnish peeling off. Again, Flagship is good in this regard, as is Captain's and a variety of other Spar Varnishes.

The best advice I can offer for varnish maintenance is keep it out of the sun as much as possible, keep moisture out of it as possible, touch up nicks & dings as soon as possible and revarnish periodically BEFORE it starts to craze and lift.
Craig

: I went with Flagship based on Z-Spar's own statement about its much higer UV
: protection. So far, so good, but I haven't used anything else, and I only
: have 2-3 months on it so far. If UV is better, what's the down side
: compared to Captain's?

: Also, how likely is a varnish/epoxy incompatibility? I was't willing to trial
: & error, anxious to get on the water, and not interested in testing,
: so I went with one of a few recommended by Pygmy (for the System Three
: they supply). I had a can of Interlux 90 a marine store assured me would
: be ok for System Three. It actually looked good in a test patch, seemed to
: bond ok, but I just couldn't face a major failure potential, and went with
: the recommendation from PYgmy.