As anyone who has a 3D printer can attest, there are visible build lines on printed objects. This is, of course, simply the nature of a machine that builds things layer by layer. A search of the web reveals a few options for removing build lines. With ABS, acetone can be used to “melt” a print smooth. However acetone if not handled properly can be disastrous. For PLA, chemical solutions are even more dangerous than acetone and probably should not be considered.
Methods that cause dissolving of a piece equates to loss of detail. Sanding is an option but also causes loss of detail. Filler primer combined with sanding, Spot putty and even Bondo have been used. These all require proper ventilation and protective gear.
I have another technique you may want to try. It harkens back to my traditional painting days. Since I now paint in Photoshop my painting tools have languished unused for some time.
I grabbed a palette knife and a jar of acrylic modeling paste and set about applying it to a part printed at .3mm layer height. I applied it much like one applies joint compound to drywall. You can also use a damp paper towel when applying it to uneven surfaces. I’ve used it on resolutions of .1mm to .3mm with excellent results.
I lay on a thin coat filling in the tiny spaces between the build lines and let it dry then sand it smooth. It adheres well to PLA (I have not yet tried it with ABS ) and is a great base for painting with acrylic media. You can colorize the modeling paste before using it by adding acrylic paint . Probably the best part is its non toxic, relatively inexpensive and cleans up with water.
In the image above the top and bottom objects were both printed at .3mm (300 microns) the bottom one has been left just as it came from the printer; the top one had modeling paste applied and then hand sanded with 220 grit sandpaper. It is extremely smooth to the touch and with the naked eye its almost impossible to notice any flaws.
Although I have not tested them yet, additional acrylic paint mediums may also prove useful. There are several Gel Mediums that should prove just as viable. They all can be colored to a desired color by adding acrylic paint before use or painted after.