Remember that most models in the 3D Ware house were created for use in visualizations, so they’re not built with 3D printing in mind.
If you import a .stl file and open it in Sketchup, there will be triangulated lines on the model. You can use the CleanUp extension to remove the unnecessary lines.
- Blender is better for organic forms (but a little more challenging to learn)
- Maya is free with an education license
- 123D Catch is free (smart phone app) – it captures 70 images of an object in the round and takes about 15 minutes to process a rendering that can be 3D printed after getting cleaned up in Meshmixer
- Meshmixer imports files from 123D Catch and you can fix/delete/mend holes