Dude,
Don't wet sand plaster!!! Start by using some medium sandpaper, like 100 grit, and scrape it with a sharp blade, alternating back and forth. That way you will avoid deep scratches.When you're close to the shape you want, scrape and use a finer sandpaper. That's how plaster masters for casting are cleaned up and polished. Plaster is a pain in the ass to work with, I used to use it to to make vac-u-form patterns with it, but it can be worked into a smooth surface with a lot of elbow grease. You can use Squadron White putty to fill in air bubbles or imperfections. Don't use anything harder than the plaster itself to fill it though, as it will not sand even. Remember when you are done, the surface is porous, so you'll need to seal it with something, like Crystal Clear, or something similar. Good luck!