After you have finished sculpting a piece, or modifying an existing piece, the next step is to sand the hell out of it. I usually start with a 100 grit sandpaper to shape the surface and remove any lumps that happened while sculpting. It is important to have a good light source, like a desk lamp, that you can hold your custom up to and look at it from multiple directions. Then I move up to a 200 grit sandpaper to even out the surface. Afterwards, I use a 300 grit sandpaper to smooth out everything, and remove the tiny scratches made by the more abrasive papers. You could stop here, but to really make that surface factory-smooth, you can graduate up to a 1000 grit sandpaper to finish polishing the surface. I started doing this recently with good results. Your local hardware store will carry the 100-300 grit sandpaper, but for the 1000 grit you will have to visit a hobby shop or buy it online. After I am done sanding, I wipe the surface off with a damp cloth to remove any excess dust before I start painting.
I usually spend more time sanding a piece than I did sculpting it. I rip pieces of sandpaper off that are about the size of a credit card. It helps to fold them in half and use this crease to get into hard-to-reach places. I have tried a few different brands, and they all work about the same. One variety pack of sandpaper should last you for at least a year.