Don't use epoxy. As stated, the sun will get to it and yellow it. Any cracks and water intrusion will blacken it. If you want to seal it before varnishing, try Clear Penetrating Epoxy Sealer CPES. First you must wood the surface (remove any coatings). Brush on CPES until the wood won't soak in anymore. Let it dry till tacky and then apply your first slightly thinned coat of varnish. The varnish will adhere to the CPES and give you a nice bond. You'll need multiple coats of varnish as the CPES is an epoxy product and UV rays will degrade it over time. I shoot for six to eight coats. The biggest difference between CPES and Epoxy is, the CPES gets soaked into the wood where it hardens the fibers. It will prevent water intrusion much better than a "coating" of epoxy. I usually do an annual coat of varnish in the spring time. If you keep up with it and repair dents and cracks quickly, you can get ten- fifteen years before you need to wood it and start over. Yes, much more time consuming that Cetol, but also much nicer looking. Hard to beat varnished teak.
Former wooden sailboat owner for 35 years.