A: You can try making an initial plaster mold, a piece mold or a waste mold. This affords you the opportunity to sand on those tricky convex curves while they are concave, and vice versa, before casting wax or another material into the plaster mold to achieve a positive. Plaster is easy to sand smoothly. Here are a few more things you could try:

1. Forget about the plastiline and use potter’s clay (without grog). This is a whole lot easier to smooth out. Use rubber potter’s “ribs” and damp sponges for the final smoothing. The main problem is keeping the thing from drying out and cracking up—especially if you’re using an armature. But misting and the use of plastic bags can slow down the drying process considerably, and the “leather hard” stage which clay goes through as it dries gives you a chance to make sharply cut edges, if called for in the design.

2. By carefully cutting curves out of flexible plastic ( I’ve used the kind that is used for blister-packs—like Avery labels come in) you can make smoothing tools that can be dragged along the contours you are trying to create. Try filing or sanding the edges to make them absolutely smooth and taper them toward the edge (if you can do it without sacrificing the curve’s accuracy.) As you drag it along the surface, you can change the bend in the tool to accomodate changes in the contours. If you make a series of these, in graduated convex and concave shapes, you can handle most of the range of troughs and humps you encounter. And you can use them like cabinet-maker’s molding scrapers as well, by cutting a profile of the desired ogee, beading, or other decorative detail into the working face.

3. Use lubrication and a piece of chamois for final smoothing. Some sculptors like to use oil, which works okay for waste-molds, but it’s not so great for rubber-molding. I like using soap for this—either liquid dish soap or tincture of green soap. The lubricant keeps the clay from smearing, which is most of the problem with plastiline. You only want to do this after you’re finished adding clay, since new clay won’t stick to the soapy surface. For really small areas, you can wrap some chamois or other light leather around an appropriately shaped modeling tool, or use pieces of a stiffer leather—I used scraps of tooling-grade leather sanded on the fuzzy side to form sharp smooth edges, and made a set of flexible smoothers in variously curved shapes. Some people favor lighter fluid and a soft brush, or wax solvents, but these can be toxic and/or flammable and I haven’t tried them.

4. You can also experiment with different densities of flexible foam. Places that set up kayaks and canoes might be willing to donate some scraps of the material they use for boat pillars and seats, which is quite a bit denser than the type used for upholstery. I’ve also used some new-type modeling tools with stiff but flexible foam tips mounted like brushes that seemed to be designed for this sort of thing. They are called “Clayshapers”. I particularly liked the flat chisel-pointed version, although there are five tip styles available in four graduated sizes, and all of them seem like they would be useful for some applications.