Mirroring an object with custom normals correctly

To mirror an object along X, Y or Z axis in Blender the "Miror" operator is usually used. For example, to mirror an object along the global X axis, in the 3D viewport window menu we need to select: Object - Miror - X Global. Or we can do it simpler just applying a scale of -1 to the object along the desired axis: s - x - -1. However, if there are custom normals on the object, they are not recalculated, which subsequently causes the appearance of various artifacts.

Read more: https://b3d.interplanety.org/en/mirroring-an-object-with-custom-normals-correctly/