I epoxied my Great Auk 5 years ago with West 105 & 207 slow hardener with UV protection.
It's been outside ever since. There may be a little clouding, but not much.
Basically, whatever is advisable is advisable, but.....you can get away with a lot for quite a while--at least five years.

I did varnish it for about a year. The varnish, highly recommended hard marine stuff, was still so much softer than the epoxy it quickly got scratched, scraped, messed up. I sanded it off and gave it a coating of epoxy about a year ago.

Long chain polymers like epoxy are essentially long molecules intertwined. (mono-filament fishing line is essentially a single continuous molecule).
UV light photons knock out electrons weakening the bonds between the polymers and the bonds in the polymer chains. Think of that cheap bucket from the 99 cent store that ripped and fractured after sitting outside for a year. There's usually a significant difference between a plastic part on your car, and that cheap bucket.

UV inhibitors absorb or lower the energy of the photons hitting the surface. Varnish, I'm guessing is a sacrificial surface. (I tried to check this out on Wikipedia and the Google, and didn't find the basic explanation that explains it more clearly. )

What I should be thinking about is, the damage is cumulative, and the epoxy continually weakens and becomes more brittle even if I don't see anything visually. What's not so bad this year, might be horrible next year. Think of the accelerating effect of epoxy setting up in reverse. Everything's fine, everything's fine, everything's OK, yech

I'm guessing what happened to Jay's paddle is less a UV effect than a thin seal coat wearing away from use. This is probably an ideal situation, making it easy to sand the surface and re-coat.

Will