[MrBasse]

As has been mentioned before, the shape will change slightly. However, the material isn't really being dissolved, it's more akin to melting. You will be left with the same amount of ABS unless a chunk drips off before you remove it, but in my experience by the time it gets to that point your part no longer resembles your part anymore...

If you orient things properly to allow gravity to pull as straight down as possible, you shouldn't have issues with maintaining tolerances on holes and locating pieces. But they definitely have a tendency to lose round a little.

Start small and play with it, it takes some effort to make a chamber that I would feel safe using around kids on account of flammability issues of acetone and the heating of said flammable liquid. On the upside, when it does start on fire it is a pleasant slow burning flame that gives you time to move before too much goes bad. That said, take a lesson from me and don't do it for the first time in front of a class full of students over carpet. Ours went fine, but we learned that this process works very quickly and continues to work after you are done with the vapor bath.

What do you want to smooth out and why? If it is just for appearance, you can't go wrong just trying it. If it requires holding dimensions it will take some trial and error to find the right times and heat, then very good consistency to repeat results.