Hi Kim...You can achieve a very smooth, almost sensuous to touch, finish by troweling on a finish plaster but working it pretty hard with a steel trowel. We make a finish plaster out of powdered clay and fine-grained silica sand, in a ratio of about 1 part clay to two parts sand. You can substitute powdered silica sand for a really smooth finish. The secret, though, in my experience is that the results come from trowel work. You need to work the finish coat, smoothing it out pretty quickly after you apply it...so don't get too far ahead of yourself, or have someone following up a person putting the finish plaster on the wall. If you make the mix a little wetter, it gives you a bit more time to work it. You can also burnish the finish plaster with a lid to a yoghurt container, describing circles with the lid.
That said, you can achieve the same results with an alis. That word, by the way, comes from alisandro, the Spanish name for the ladies who used to do the plaster work. We make alises with a 50:50 mix of powdered clay and fine-grained silica sand. Instead of adding water, we use powdered milk, diluted by 50%. In other words, add twice as much water as the recipe for preparing "drinkable" mild.
Be sure to experiment with mixes and techniques before you start working on a wall so you know what you are doing and can do a superb job on the finished wall. Keep in mind, too, that it takes a long time to really master finish coats. I still feel like I'm learning and I've been at it for a lot of year.
Hope this helps.