Can we assume you are working with a machinable (hard) wax? If so,
there are a number of ways to smooth the wax. Depending on what kind
of surface you have, and what kind of detail will help determine what
method works best for you.
You can use abrasion, heat or flame, solvents, or a filler.
Experiment with everything.
A scraper, made of a knife-edge graver, or other similar sharp
shapes, works great to smooth down broad surfaces, like pealing
potatos, or a flat-bottomed graver works well in a pulled motion,
like drawing, on smaller areas to smooth rough surfaces.
You can try a flame or a heated air tool to "melt" the surface of
the wax slightly, but be careful with this, as you can melt detail
quickly. Sanding sticks of assorted grits will work well with hard
waxes, and the application of a wax solvent with the finer grits will
do a terrific job of smoothing surfaces. Solvents can also be applied
with the traditional piece of nylon stocking, as well.
Patch-Eze, or Disclosing wax, made for the dental industry, is a
great filler for smoothing out small abrasions or pits in wax before
casting. I usually use a clean finger to apply and smooth it before
investing my models.
Jay Whaley UCSD Craft Center