In my experience, Sketchup consistently produces poor STL meshes. The only solutions I've found are to import the meshes into Blender and repair them manually,

In my experience, Sketchup consistently produces poor STL meshes. The only solutions I've found are to import the meshes into Blender and repair them manually, or redesign the project in a different CAD program. (I've settled on Solvespace for my personal use.)