For complete transparency, surface treatment is required to smooth out the surface in order to create a transparent 3D print. There are two separate methods to achieve this.
The first method involves using a solvent to smooth the print in order to remove the appearance of layer lines. For instance, PolySmooth filament, a filament type designed specifically for models which are intended to be transparent, can be smoothed using ethanol. The ethanol is sprayed onto the print and left to dry. After several reapplications, the part will turn transparent, but the part may lose dimensional stability as a result of the application of a solvent to the print.
In addition, it takes a long time (days) to dry completely. Finally, only PolySmooth filament can be used this way. Other forms of solvent finishing tend to fog up other plastics such as acetone smoothing on transparent ABS.
Another method involves coating the print using either XTC 3D or a spray-on polyurethane coating. These coatings are applied several times until the surface of the print becomes smooth and transparent. This is similar to painting an object; like the solvent method, however, it can change the dimensions of the part.
Both of these methods work by smoothing the surfaces of the print until the part becomes transparent. The only difference is whether material is added or removed.