I do from time to time like to model railway themed dioramas. My question is when it comes to applying a primer is it best to add a red oxide to parts of the kits that would be metal.
I’d just use the primer that is appropriate for the top colour being used. You are not trying to stop the model rusting, which was the original reason for a red lead/red oxide primer. In most cases mid grey works well for me. The exception is under a red top coat. I would use red oxide for that.