TL;DR of the post, generated automatically.

OP wants to fix the normals and possibly poor geometry in a shield model before texturing it in Substance Painter.

• Symptom: With Face Orientation turned on, OP sees many red faces.
• Suspected effect: OP thinks these faces are why the middle of the model is invisible in Substance Painter.
• Materials: Five screenshots are attached, and the file is included as Shield15.blend.