I am a total beginner with 3D printing, I have my Ender3 (with Cura) for about two weeks. So I have a lot of questions, and this is first one: I try to print an F18 model, as lightweight as possible, so there is no infill and the shell is 0.4mm (the size of hotend). It basically goes OK, but the surface has strange bumps on some places. What setting should I adjust and how?