How to smooth out a stucco exterior?
I would pressure wash the entire surface with plain water. Make sure to keep about 18 inches away from the surface and use a wide fan tip. You want to wash it well, but you don't want to peel off any of the material. Allow the surface to dry for 2-3 days. 

If the stucco is structurally sound and there is no loose or peeling paint, you can use a bonding agent such as Weld-Crete. Then you can apply new stucco right over the top of your existing finish.

Kirk Giordano is one of the country's greatest plastering companies and he has many videos that detail how to get a variety of finishes.