You can spend hours sanding and polishing clear PLA 3D prints and never get to a real transparent finish. We’ve started experimenting with a product called XTC-3D, a resin which is just viscous enough to fill all of those horizontal layer gaps without globbing up on the top. it results in a much clearer finish without additional work. It’s a big deal for us as we’re dealing with a lot of LED related research projects. Object walls have to be very thing to achieve this effect, and infill will ruin the transparency since you cant apply the resin to the inside walls.
https://www.smooth-on.com/product-line/xtc-3d/
You can also use it on non-clear items to save time on smoothing, with a light sanding you can then paint right on top of the finish without needing a filling primer.