Use rakes they are the clay equivalent of sandpaper. Start with large ones, then work your way down to finer smaller ones, once you have the surface very fine, go over it with a wide paint brush and baby powder then Polish the surface with a soft cloth. It takes a very long time. Soft clay is not really the best choice for sculpting mechanical looking parts, harder clays and firm waxes are generally used for that, Im not saying it cant be done though.
For oil based clay use a little paint thinner, Napthanol, or Acetone with the use of brushes, soft sponges (the kind they sell in those pottery tool kits), or a scotch brite pad for really aggresive material removal.
The solvent eats away at the oil based clay and evaporates the way that water does with water-based clay. It will not change the overall consistency of the clay, it will be pretty much the same after the solvent evaporates.
and if you can get paint thinners ive had good success with lighter fluid. you can mix up a realy watery version by using 1:2 wax and lighter fluid. its prity handy for filling scraped areas and will return to normal after its evaperated.
If you are finishing off your sculpture and are done smoothing it, you can clear coat the clay to give it a slightly hard shell. The clear coat will also smoothen out some of the imperfections. Crystal Clear.
so far my method has worked really good,
it's basically "sanding" the clay, just use computer paper like you would sand paper, just don't press to hard and tear the edges or that will just displace the clay. Tear the edges so they don't accidentally carve into the clay. As you can see on my avatar, it gave me good near perfect smoothnes.
Anyhow, like i said, don't press to hard, cuz then the paper doesn't work as an abrasive, it just moves the clay.
If you go lightly, it actually takes clay off and smoothes it, not a significant amount taken off but smoothes it just fine.
i'll have to give that Rake and baby powder thing a try, how do you do it? and does it yeild as good results as my technique (as seen in my avatar) put out? I am really interested in learning what you are explaining,
(the car is clay, no its not mine hahahaha)
This a little blurb from a website on the subject of tooling clay properly.
Once you have a final form with all the details carved in- you will need to develop the clay surface for painting or to take molds from. There are several stages in developing the surface, each of which improve upon the last stage, almost like grades of sandpaper.
Finishers are similar to rakes with smooth edges. Used in a criss-cross motion across the clay surface, they smooth the imperfections left behind by the rakes.
Steels are the last tool to actually cut the clay. Quite simply, a steel is a sheet of Spring Steel which is available in many different gauges down to small fractions of a millimeter. Work your way down through the gauges until it is hardly removing any clay. Steels can be found in many shapes and sizes or you can make your own from a sheet of Shim stock steel to suit your job. Basic shim stock is available from good tool supply companies. In the US, McMaster-Carrs shim stock assortment- Part Number 9300K27 - .003 Feeler Gauge stock 2093A16 is recommended. You can download steel templates from Chavant
Slicks are the final tool to bring out the surface of the clay. Slicks are made from sheet Lexian (plastic) with polished edges to burnish the surface of the clay. Again they are available in many shapes and sizes to suit your model. It is possible, using slicks, to get a surface that you can see your face in."
The only thing different in that process to what I was taught is using slicks at the end, slicks are great for large smooth flat areas (like on car models) for smaller detailed parts, I use baby powder (stops the surface grabbing) a soft wide paint brush then rub with a soft cloth, but you need to rake it down before doing this, unless you're sculpting organic forms like skin textures
I am really interested in learning what you are explaining,
I just did a small scale test of the baby power with the rakes and it seems to do a pretty good job, but you have to have your clay already pretty level for it to work.
Anyhow, it works pretty well, i think i might try it next with my method aswell, babypowder+paper sheets like sand paper.