Ultra-violet light, especially sunlight, causes finishes to deteriorate. Many finishes sold at marinas and home centers claim UV resistance, but those sold at marinas are much more expensive. If both work the same, why would you pay more at the marinas?
So I did a test to compare the UV resistance of the varnishes from regular paint stores and home centers with those from a marina.
For the test, I stained a panel with a red dye because red dye fades relatively rapidly in sunlight. Then I applied five coats of four different varnishes to four sections on the panel and placed the panel in a west-facing window for six months.
The conditions (through glass and exposed just during the afternoons) were not nearly as harsh as those on a boat or on a project kept outdoors. But I reasoned that it shouldn’t take long to determine if UV light was getting through, because the red dye would fade.
The varnishes I used, as seen on the photo of the panel, were (on the left) Z-Spar, a common varnish sold at marinas, two common exterior, supposedly UV-resistant varnishes, sold at paint stores and home centers, and a common indoor varnish that makes no claim of UV resistance (on the right).
I covered the top two-thirds of the panel so that only the bottom third was exposed.
As you can see, the marina-bought varnish did its job well. But the two home-center varnishes did little better than an interior varnish with no UV-resistant additives.
My conclusion is that if you want a varnish that will hold up best for outdoor projects, buy the varnish from a marina, many of which have websites and shipping capabilities.