It is very similar to paper packing material but if you're using a plastic mold it actually comes out surprisingly smooth. I use Durham's Water Putty, which is a strengthened plaster, to fill surface pits and then spray varnish to coat the surface to make it more plastic-like. The only issue I could see with doing pieces like flying saucers would be a slight warping of the surface that happens as the shape dries. But this can be avoided somewhat by slower drying.
Like dijit said, car body filler and sandpaper can really work magic.
Years ago I was contacted about making large, 3'-4', flying saucers and started working on plans to build a hot-wire lathe to turn insulation foam. Depending on how big you need the final saucers to be you make look into something like that.