Normally, I use the accompanying photo to show that mass-marketed varnishes sold in home centers and most paint stores for exterior finishing don’t contain any more UV resistance than common interior varnishes. But the panel also shows that even the much more expensive varnishes sold in marinas still aren’t perfect at resisting UV light.
I stained the panel with red dye (because red dye fades faster than most other colors) and coated the panel with five coats of four different varnishes. From left to right, a popular boat varnish from a marina, two common varnishes sold in home centers for exterior use, and an interior varnish that wouldn’t be expected to contain UV-resistant additives.
Then I covered the top two-thirds of the panel and placed it in a west-facing window for six months (through the winter, actually, to avoid tree leaves blocking some of the sun’s light).
The purpose was to show that mass-marketed exterior varnishes aren’t any better at blocking UV light than interior varnishes. The red dye under the two exterior varnishes in the middle have faded just about as much as the dye under the interior varnish.
But if you look closely at the left side of the panel (Z-Spar), you’ll notice that the red dye on exposed part is also beginning to fade. Even the best UV-resistant varnishes aren’t perfect.
This is the reason boat owners with exposed wood have to sand back and recoat so often. We don’t know how to make a clear, transparent coating that can stop the penetration of destructive ultra-violet light. Pigment (paint) is required to achieve this.
The lesson is that if you are clear finishing an object that will be exposed to sunlight, either outdoors or indoors through a window, you cannot expect to totally avoid UV damage no matter which finish you use. You can only slow the damage from occurring.