Smoothable filaments are 3D printing filaments that can be polished to give a layer-free finish. This is a mostly aesthetic quality that gives a smooth, mirror like finish. In order to achieve this, some degree of post-processing will be required. The amount of post-processing will depend on the finish that you want and the specific filament that you use. Always check the Technical Data Sheet for specific information on polishing.
Smoothing may require the use of a specifically designed machine or it may be possible to smooth the product by hand. Whilst most plastics used in 3D printing can be sanded or polished to some extent, smoothable filaments are intentionally engineered to be finished with no visible layers.
PVB is a plastic that is commonly used for smoothable 3D printing filament as it is possible to achieve excellent levels of clarity whilst maintaining great physical performance.