Although we only use the best materials: Select Cedar, Select Fir, Select Pine, outdoor polyurethane with UV protection and water proof glues, all materials are subject to wear from the weather. Therefore, if you want your sign to last you need to give it a light sanding and two to three more coats of polyurethane once a year or when you see the clear finish has worn away in any spots.  Graying or fading of the wood means it has lost it's protection and damage is occurring. The amount of maintenance required depends on the amount of sun and weather it receives. Modern finishes are designed to wear away rather then flake and blister.

All wood requires protection from the elements. Cedar is more resistant to decay then other woods if unprotected but if any wood is protected and that protection is maintained, it will last.

I recommend using: A good Polyurethane Exterior Water Based - Semi-Gloss.

Is epoxy better then polyurethane?

Epoxy is tricky to work with. If you are skilled with it you can do it but you can easily mess up. Epoxy paints are more rigid, hard, and brittle. So, it will crack if exposed to temperature changes. Polyurethane is more flexible than epoxy because it has plasticizers in the formula. Its plastic nature makes it stretch and contract based on temperature changes. This gives it improved flexibility.

So for outdoor, polyurethane is better than epoxy because it is flexible and can cope with temperature changes. For a more lasting finish, do 5 coats.

Check out these references:

11 Best Clear Cedar Finish for Outdoors ( 2023 Reviews) (woodworkingclarity.com)

Polyurethane vs. Epoxy (What's The Difference?) - DIY Geeks