A better option is cellulose fiber board, commonly known by the brand name Homasote or sometimes "sound board". The common version is Homasote 440 SoundBarrier, which comes in 4 x 8 ft, ½-inch sheets, although I have seen it sold in half sheets or quarter sheets as well. When I was in art school the hallways were lined with these boards to attach art work. The common type is not completely smooth, but you can finish it smooth or cover it with fabric. It's $26/sheet at my local store.
Homasote also has a product called PINnacle, which is marketed specifically for pin boards. I don't know how different it is from SoundBarrier 440; it may be smoother.