What kind of cleaning are you trying to accomplish?
If you just want to remove dirt and debris, use a nylon bristle brush. If you want to remove loose splinters from the surface, use a brass wire brush. If you want to scrub the surface down to accept stain/finish without completely smoothing out the original texture, use a steel wire brush.
You can get brushes like these at any hardware store
If you do need to sand the surface smooth, consider a sanding sponge:
This will wrap around the curves and ride over the bumps and won't tear up like sandpaper. You can also wet sand with it.