Oh, sandpapers do leave scratches. but that's the point. You scratch them uniformly.As for smoothing, i tried sandpaper on some scraps but it leaves nasty scratches. Thus far the best method seems to be a wet blunt sculpting tool, sort of smear it even. Gloss varnish might also give a more 'finished' look as well as hide any slight bumps and such.
I never used fancy clay like yours, so I don't know whether it's similar with normal clay or not, but from what little thing I remember from when I work with clay, I sandpaper them a lot. Really lot. I mean, sandpaper like crazy until all the tiny bumps are gone and the outer layer of the clay is uniformly missing (I mean, you don't even see the 'skin' of the clay anymore. That's what I meant by uniformly scratched), and then use gradually softer sandpaper until the softest sandpaper until all the scratches are gone. The colour would look different compared to unsandpapered clay, because the outer skin that touches the air directly when it was drying isn't there anymore.. It does take a lot of time though. I'm not kidding. And as I said, I don't know whether your clay works similarly as mine. Maybe you could try it on some scraps.