Teresa, the implement paint doesn't have to be sealed. It's not sealed when used on tractors or vehicles and stays on for years. It will probably outlast the signboard.
Natural rocks are so porous they hold just about any kind of paint for several years without sealing. All the seal does is make it adhere to a surface better. But if it's patio paint, like Boo mentioned, and if the surface is fairly porous, the patio paint doesn't need a seal either unless it makes you more comfortable to use it. An outdoor type seal won't hurt anything, and might make it last a year or two longer.
