What can be done to create smooth vertical walls

I am wanting to fabricate magnetic hex tiles for a board game. I want to make the pieces out of 2 clam shells so I can insert the magnets in the middle then glue them together. I need the large flat surfaces to be smooth so I can put decals on them for the game. I have tried 3d printing them before, but I always run into this issue where the vertical walls are bumpy because of the layers, so when you try to assemble the board the pieces catch on each other and do not lay flat.

Is there any way to fix this on a FDM style printer? How smooth are surfaces (not directly on the build plate) of Resin printers?

Any advice is appreciated, thanks!
John Vickers