For most project signs we use a material called "Omegaboard". Is is an exterior sign grade plywood with a smooth vinyl laminate on the face. The result is a glass smooth finish with a nice gloss laminate on top. A very classy high quality project sign!
We also use quite a bit of aluminum for a more permanent signs. The background is an aluminum PVC laminated panel to
minimize any "Oil Can" effect. It stays nice and flat with the longevity and non rusting properties of aluminum.
We use vinyl fence poles for most installations of a permanent nature. These are an extremely long lasting no maintenance product as any Vinyl Fence guy will tell you.