Selecting Back-Facing Triangles
Typically, any time the Unreal Engine renders a Static Mesh, it only draws the triangles whose primary faces are visible to the camera. All triangles that face away from the camera are culled — that is, removed from the scene — in order to increase rendering performance. However, some modeling applications do not make the same clear distinction between the front face and the back face of a triangle. Therefore, when you import Static Meshes from those applications, you may end up with triangles that are not rendered where you expect them to be, or extra triangles that are rendered at unexpected angles. For example, the base of this cylinder is facing toward the interior of the object:
When you enter Edit Mode, the Static Mesh Editor viewport overlays your model with a wireframe view of all triangles that are currently visible to the camera: both the ones that face the camera and the ones that face away from the camera:
By default, you can only select triangles that are facing toward the camera. However, you can enable the Backfaces toggle button in the Mesh Editing Toolbar.
When this setting is enabled, you can select triangles in the viewport of the Static Mesh Editor even when they are facing away from the camera:
This setting can make it much easier to select the back-facing triangles in your model, so that you can use the Flip command to make them point in the opposite direction.