Smooth PLA 3D printed parts. Using tetrahydrofuran, or THF, as a solvent for PLA. suspend the print in a glass container, pour in a tiny amount of the solvent, and (gently) heat it. The evaporated solvent will smooth all the ridges out of the print, leaving a shiny and smooth surface. You can, of course, hand polish it by dedicating a lint-free cloth and a pair of gloves to the task.
It is a hopeful development when innovation, community, and environmental improvement dovetail; the project Perpetual Plastic which uses plastic garbage to make 'ink' for printers may be considered one such development.