Clear resin 3d printing has advanced in the last few years and can be very useful for creating space models and crude optical prototypes. However, the problems of striations/Layers does limit the optical appearance and performance of 3D printed prototypes. Even for micron-scale layer dimensions (should 3d printed technology ever achieve that precision); such micro structures would actually diffract light over a large angle. Clear lacquer coatings are used post process to coat the 3d printed parts and help mask the appearance of the surface layers but again such a coating does not fully eliminate the texture.
UK Optical plastics has long experience in producing optical prototypes both refractive and reflective. For high quality refractive prototypes in Pmma or Polycarbonate. UK Optical plastics has developed a number of in-house techniques including CNC machining and special polishing techniques to provide high quality surfaces and optics.