Making something solid

I am new to this and am having difficulty with the solid part of the design. I have created this part and would like to try 3d printing it eventually but first it has to be solid which it is not. I have grouped it but that doesn’t make it solid. What has to be done to make an item solid?

Coach.skp (240.7 KB)

To be considered solid and 3D printable, the walls have to have some thinckness. Your walls have no thickness. They are just the outer skin.

The requirement for a solid is that every edge is shared by exactly two faces. No more and no less. You would need to do something with the windows as well. Either add a skin across the opening to represent glass or create the inside face for the walls with the openings where the class would be.

Compare this to your model. The walls have thickness and all are complete. The arches at the ends are not separate parts like you had them and the whole thing makes a solid group and should be printable.
Coach.skp (154.7 KB)

I assume this is for a model railway coach. You would be wise to work with the units set to meters instead of millimeters. Then the walls at 1mm thick would be drawn as 1 meter thick. This will help get away from issues with tiny faces.

Thanks, I made the whole walls and end by extruding a rectangle then hollowing out the inside. I believe it got modified when I was doing the windows, I’ll try again.

Another question, the reason I used mm is that this is the actual size of the model, if I use meters then how do I scale it for printing?
Thanks again,

You’ll export an STL file from SketchUp to use for printing. STL files are unitless. You have to tell the slicer software what the units are so there’s no need to scale the model down before exporting the STL file.

Added note, you had the windows grouped as well as those little arched bits for the roof. Those window groups need to be exploded as well. Again, examine my version of the model.

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For a railway coach-sized object, the best option in my opinion is to model it to full, real world scale, and to reduce the size to model railway scale in your 3D printer software.