What happens over time is sort of like asphalt paving on a street. At first it looks really nice, smooth & tight, good surface & no problems. Move along a few years and the sun, wind & rain begin to come into play. Any small crack or fissure becomes magnified by temperature & humidity. If any moisture gets into the wood, localized swelling will be the result. The fibers soak water, the wood expands and the surface is lifted. Any finish is now functionally destroyed in that spot and things begin to break down rapidly at this point.
The easiest solution is a good sealing job at the time of the build (prevention). That's why aluminum skins work so well to protect things. The aluminum can only leak at an edge or fastening, otherwise it just isn't possible. Epoxy & glass is also good, but a UV coating is still needed, because the sun will eat epoxy over time. Paint on canvas or some other fabric works well, as long as the paint is intact there will be no change under it. Etc. etc. etc.....