We use 2 methods to smooth the clay. We use a lot of metal scrappers, keep scraping over the same spot from different directions again and again and again. Running your hands over the curve/surface to notice imperfections and problems.. more clay and more scrapping. Work the surfaces and the edges will fix themselves. Once your happy youve taken that method as far as it can go, we very lightly sponge (not brush) on a layer of White Spirit. This has the effect of melting the very top layer of clay smoothing it out more. Id only recommend doing this once you are happy with your sculpt.