I've used Acetone a few times. I didn't quite get the results I saw in a video that gave me the idea but it sorta worked. I'm sure you could get it to work if you take it slower than I did. What happened with me is it sort of turns the sculpey into a sludge after a while and that was somewhat problematic. Think I just overdid the amount of time and acetone...at any rate, I generally use water (spit) and a colour shaper (soft silicone tipped type of brush) or my rubber-gloved finger for really large areas. I think the trick is to work up to it in stages, like sanding, you can't go from 80 grit to 1200 grit and expect it to be mirror smooth without taking a lifetime working the 1200 grit. In other words, it's got to be fairly smooth before you get to the final smoothing stages. Having said that, you can sand it afterwards, that's always an option.
That's a cool sculpt, what scale or size is it?