Medium Density Overlay plywood is
widely used for signs in 1/2" or 3/4" thickness. Also
called Crezon, it is covered with a bonded layer of a stiff paper product providing for a smooth, longer lasting surface. It is then
primed & painted. (Regular plywood is generally not used for signs).
For even more longevity for a sign, one can opt for MDO-A, an MDO
board with a bonded aluminum surface, finished with a super
durable white baked-enamel.
Some samples of plywood signs: