Don't know if you are even considering a varnish product.
But if so, you could consider a spar urethane.
Somewhat unrelated but I use it on all of our wood, interior trim on our windows. I am not disappointed.
Google best finish for outdoor wood products.
Spar Urethane
Spar urethane is a clear, protective finish that is resistant to water, UV rays, and temperature changes. It is ideal for outdoor furniture that is exposed to extreme weather conditions. However, it can be difficult to apply and may require sanding between coats. Examples of spar urethane include Minwax Helmsman and Rust-Oleum Marine Coatings.