Unfortunately, for cosplay/prop building, sanding is the most reliable way to smooth out those layer lines. Even if you use some sort of filler (like Bondo or wood filler), you’ll still need to sand it. Even at lower layer heights, you’ll still have to sand it – I print everything at .16mm and I still have sanding to do.
You can use electric sanders (I highly recommend using ones with variable speed), like random orbitals or even handheld belt sanders like this to make it easier. That said, some parts will require you to just do it by hand, which is the most reliable way.
