Smooth 3-D printed parts are difficult because of the way most 3D printers print. Each layer builds up on the previous layer so the printed part is a combination of lines or ridges. Smooth 3D printed parts make smoother, more finished castings. Whether you are making more parts in plastic, soaps, candles, or chocolate; smoother master parts will help.
For acrylic 3D printed parts or ABS plastic, use acetone that you can find at the hardware store. It is also sold as nail polish (amazing to me that acetone is acceptable for putting on your hands!). Acetone smooths your part by basically putting the ABS into solution, and then drying away, leaving a smoother finish.
So to use, just smooth on the acetone onto your 3D printed part. You can do this by dipping, brushing, wiping. Rubber gloves is highly recommended. And definitely do not breathe the fumes of the acetone. It is not healthy for long term exposure.
If you dip your part in the acetone, as we do in the video below, be careful about leaving the acetone inside the 3D part. If the acetone cannot evaporate, it will continue to dissolve your creation.
You can also use your covered fingers to smooth out the ridges.