Sometimes you may import a model that has had some of its faces disappear. This might be because of the incorrect normal direction. For example; no matter how you adjust the perspective of the camera, sometimes you might be able to see the inside of a box without being covered by the outside of the box. For this, you may use the Flip feature to rotate the faces inside out.
Load a model. In this case, the hood is missing because of the opposite direction of the normal.
In the Tool Bar, press the 2-Side Picking 
tool. If you do not enable this tool,
then you will not be able to pick any face from its back side.
Select the mesh nodes that disappeared in the tree view.
Click the Flip button in the Normal section.
Tips:
This feature is especially useful when you want to create a dome-like environment, or terrain, where the camera focuses on the scene.
See Unifying the Normal if some of the faces fail to be rendered with the other faces, causing holes or cavities on the surface of the objects.