Some models require rounded surfaces. I've tried making prisms with flat surfaces, then rubbing into them with something round to make the surfaces rounded. Is there a better way to do it? How can I do it for spherical surfaces? I'm talking of something like a ball or a tyre. Then there's the task of making a model that's prismatic on one side and cylindrical on the other. How can I do this? Something like a pencil or an engine of a fighter aircraft.