Epoxy resin should not be left uncoated when it will be exposed to sunlight. It suffers actinic degradation i.e. it breaks down due to the action of the ultra violet component in natural light.
When used in marine applications (I've done a lot with full size boats) it is over coated with polyurethane varnish which includes elements which suppress UV degradation (usually cobalt based).
If you don't overcoat it, it will yellow and age quickly.
Cured epoxy should also be wiped down with acetone or white spirit to remove the waxy deposit which is exuded onto the surface as it cures. This 'amine blush' as it is called, will prevent proper adhesion of the varnish unless it is removed.
I've used 'West epoxy' extensively in the restoration of sailing boats, and also use it to fuelproof the firewall areas and engine bays of my models.