I've made a discovery recently. A coworker was playing with "transparent" ABS filament and I decided to get some myself and see just what it was capable of. And while the filament itself is clearly (heh) transparent, prints always turned out milky. I decided to investigate why.
The first thing I printed was a filament holder to hold the spool of transparent filament on my filament rod. When the print was done the bottom where the print contacted the glass was mostly transparent, the infill pattern could be seen easily through the print wall, the rest was just a crystal white. Even after vapor smoothing it the outside just got real shiny, but no more see through. But I assumed that was just because the vapor smoothing only smoothed the outside layers lines and the layer lines inside the print on the other side of the shells were distorting the print.
I've seen bubbles on smoothed parts before. If you leave a part in the acetone vapor too long the surface gets bubbly. I never knew why, but now I have a theory. When printing the extruded plastic gets squished, but it's not flat on the sides, it's rounded. This means air gets trapped between the lines. Not much, just enough to look like internal scratches but enough to turn a transparent 100% filled print look milky. But when vapor smoothed the plastic becomes liquid and the micro-bubbles join together to form bigger bubbles and make their way to the surface.
I attempted to decrease the extrusion width in simplify 3D to 0.3mm to try and trick the slicer into packing the fill even tighter. The result was slightly less of the internal scratches, but they were still there.
Conclusion, while the transparent filament is pretty, the FFF printing process makes it not suitable for making actual transparent objects, even with smoothing. It might be possible for single wall objects, but you have to be very careful with the smoothing or you could end up melting your print.