I’m having a huge amount of trouble figuring this out. Luban randomly adds a skin layer top surface over spaces that should be empty, holes… There are clearly holes there in the STL, and Luban even renders it correctly, but the moment I generate the G-Code, the hole is covered up with a yellow layer. It’s so bad that I have to slice what should be a monolithic piece into up to 6 pieces to eliminate the problem, and then glue it together.
That usually means your mesh isn’t correct, the vertices don’t line up correctly. There’s probably some random vertex off in left field that causes Luban to render a large flat plan.
Search the forum for repairing meshes. I’m not good at it though. I usually just start over from scratch and redesign my (simple) pieces.