One must remember that the bluestones are harder than granite, so why are the bluestones smooth and not the much softer stones made of sandstone ?
I don't know about "smoothed" but the sarsens were certainly "dressed" - and this was done to the outer faces rather than the inner ones. And sarsen isn't soft sandstone its incredibly hard.Incidentally it can be smoothed quite easily to a beautiful shiny finish using salt and a piece of flint though I guess that would weather away over time.