Carved or sandblasted signs do not necessarily have to be made of wood, but can be realized from closed cell urethane foam. Foam has a very good outdoor life span with minimum maintenance, but has its limitations being a different specific gravity than that of wood. It has less longitudinal strength compared to most wood over a same length, with 50% more liability to denting. However, foam works better than wood on certain applications if it is designed and installed properly.
Wood would be our suggestion for large hanging or ground signs, similarly a large foam sign fairs best fastened to a wall for needed support against whipping and flexing in the wind. Also with a foam sign, the reverse side against a wall is not as moisture absorbent as wood, therefore does not require the same maintenance from rot providing good drainage is maintained from proper installation.
All signs from our sign shop have minimum five coats of paint including primer, with either a gloss, or egg-shell finish. Outdoor life expectancy on most painted signs range from 3 to 5 years without needing re-coat. The sign color, weather and amount of sun it receives daily affect every application differently. A natural or visible wood grain finish needs more maintenance than a pigmented finish coat.
Another consideration for sign location/maintenance would be its proximity to conifers. Acidity from conifers has more of a detrimental effect to paint than most other trees, however any leaf sap is going to discolor the sign and increase its need for maintenance.