I find that when I want a uniform smooth surface in clay first I work it with toothed tools handled metal toothed/leaf-shaped wax scrapers, flexible stainless flat scrapers with teeth, hacksaw blades, etc.
Seeing the furrows and how they follow (or don't) the intended surface contours allows me to add bits of clay where there are hollows, and trim-off high points or bumps that don't belong. Only after the surface looks uniform and regular with all these little furrowed surfaces do I come in with the smooth-edge of the (stainless, plastic, or rubber) flat scraper and lightly remove the texture.
The texture is not removed all at once, but in multiple very light-pressure passes made from as many different directions as possible given the form these changes of direction in attack prevents irregularities from developing with each subsequent pass of the scraper. For best results the scraper is held loosely and allowed to follow the contour of the surface with a little bit of spring in the grip if held too tightly or applied with too much pressure the subtle degrees of control are lost, thus resulting in irregular surfaces.
Each type of flat flexible scraper stainless, plastic, or rubber will work best on clay that is in a corresponding degree of relative 'hardness.' By that I mean that I find hard stiff plasticene is best worked with the stainless scraper, medium grade plasticene gives best results when worked with a plastic scraper, and very soft plastic plasticene is best worked with a soft flexible rubber scraper.
For what it is worth, those car body models the auto manufacturers use in the initial stages of 3D design are made out of very hard/stiff plasticene and worked almost entirely with scraping/cutting actions.