I haven't done much coldworking on my kilnworked pieces as of yet. But, after seeing several artists pieces that have extensive coldworking involved, I want to move up myself. I am currently looking at sandblasters for starters, but which next- hand held wet sander, wet belt grinder or flat lap. I know the flat lap would be the most expensive, and not everybody can make a bowl look as good as Marty. So let here your suggestions.
If your flat surfaces are small, a few square inches or so, you can get(or make) a flat plate to lie behind the belt, instead of a platen, for a belt sander. This allows you to do flat on a belt sander, albeit on a small scale. I have one that I use rarely, since I have a lap wheel too, but it is handy for certain situations, and if you're on a budget and working small it might do the trick.