PVB (Polyvinyl Butyral) is an alcohol soluble filament that can be smoothed by using isopropyl alcohol. PVB is easy to print and it has a clear appearance when printed. 

To smooth the surface of your 3d print, you can spray 99% isopropyl alcohol on the surface which will slightly "melt" the layers together. The result is a smooth finish and in most cases can make the 3d print watertight.

You can also use an alcohol vapor chamber, such as the Polysher by PolyMaker, to smooth your prints. It is recommended to let your print dry for 24-48 between sprays.