I have been working on costume parts, my process with split pieces is first assemble with painter’s tape to get a test fit, disassemble, super glue the pieces together with locktite gel control, sand with 80 and then 220 grit, paint with rustoleum 2-in-1 sandable filler, fill in gaps and seams with bondo glazing spot putty, sand the entire thing with 220, and then repeat paint-bondo-sand until I am happy with the surface before I move onto actually painting.

Basically this process.

If I don’t want to paint something, and don’t care too much about layer lines, I’ll usually use super glue and then a wood burner to melt the edges together. It isn’t the prettiest, but it works fine.