Getting rid of print lines can be the most annoying thing ever... but I've come up with some pretty good methods over the years. Firstly if I'm printing something that I want to have a super smooth finish I will go with ABS over PLA, I know that ABS can be a bit tricky to print with because of warping but it's much easier to smooth out than PLA.
So this mask was printed in ABS with .3 micron resolution and 3 "shell" walls, the 3 walls is super important, any less and you'll sand right through the outer layer of your print in no time! After everything printed I did a light sanding of all the pieces with a medium grit sandpaper and then I applied an ABS and Acetone mixture over all the pieces. This mixture is the secret to smoothing out ABS prints, grab some acetone from the hardware store and a few acetone resistant bottles from Amazon (I got some small nalgene ones that are great). Since ABS is acetone based it will dissolve the ABS, so mix in a few pieces of scrap ABS with some acetone and wait a few hours for it to disolve completely. The ratio of acetone to ABS is really an eyeballing kind of deal, I just add a few pieces at a time until the consistency is similar to milk.
You can then use natural hair brushes to apply the mixture to your pieces, make sure to not get any synthetic hair brushes as the acetone may melt the bristles. When applying the mixture to your prints be sure to wear gloves and a respirator as Acetone is quite toxic. When applying the mixture to the models it's also important that you work fast and don't focus on any single area for to long, acetone evaporates very quickly and the mixture will start to dry as its applied. You'll eventually get a good eye for applying the mixture, practice makes perfect!
After the mixture is applied you can slightly sand everything down, working through the grits as you go to get a smooth finish. After that you can either apply another layer of the slurry mix or if the prints looking good you can move on to priming and sanding. I usually do 2-3 passes of priming/sanding, finishing with primer.
Hope this is helpful!
