Best tip I can give is on speeding up the cure. Normally it takes a good 24 hours or more to fully cure, but if you put it out in the summer sun (I was living in southern Missouri at the time so probably 90-95F outside) It will cure in at most a couple hours. Alternatively you can put it under a heat lamp to speed the cure if you need to.
It can be smoothed with water, no problem. Gives you plenty of work time as long as it's not too hot out.
Also, try to get it as smooth as you can before it sets.. after it sets rasps or like 60grit or lower sandpaper on an orbital sander, are your best bet to start, because the stuff is pretty hard to sand once it's fully cured.
Being an epoxy clay it is incredibly strong, just be careful because it can build up weight extremely fast.