3.1. Remove intersections
While designing, try to have a consistent modeling approach and make sure you do not leave any faces or edges inside the volumes. In case you do, you can delete them by selecting the edge or the face and use the Delete key on your keyboard.
3.2. Reverse the faces
When 3D modeling, all faces need to be properly oriented with their front faces visible. To quickly correct a model with improperly oriented faces, you can right click on them and reverse them.
3.3. Non-manifold edges
While 3D modeling, it may happen that you create an edge on your mesh belonging to a single face on your model. This means your mesh is non-manifold and thus, it cannot be 3D printed. We advise you to go back to the modeling phase and delete the unwanted edges.
3.4. Multi-part files
This is the case when your file is a 3D model which consists of assemblies and you want to 3D print it.
To repeat files (multiple copies of the same model), the most economical technique is to use Sculpteo's online 3D printing tool which creates a series (of at least 20 pieces), which optimises the positioning and price of production.
3.5. Apply Colors and Textures to the Front Faces
You can apply colors and textures with Wings 3D tutorial. Though, you must be aware that some export formats take into account the 3D geometry only.