Unfortunately with FDM you can’t get it perfect but can be reasonable, with SLA and resin it’s better.
If you want a try at it in FDM there’s a few things you can do:
- Clear PETG seems to be better than clear PLA.
- Use vase mode if possible (although this is not always suitable for models due to strength etc)
- if not using vase mode use 100% infill and make sure the infill pattern is the same direction each layer to reduce internal refraction.
- Experiment with bigger nozzle sizes and thicker layers. That layer itself will be clear, it’s the bit between layers that causes it to be less transparent.
- use smooth sheet (no patterns or surface finishes) and print part at right orientation, the X axis will be most transparent.
- you can wet sand the fdm part after with various ranges of grit to achieve even more transparency.
Not sure on the yellowing of filament sorry.
