Unifying the Normal
If you use Flip function and you still fail to mend the holes or cavities on the surface, you might need to apply the feature in this section. Unifying the normal adjusts all the normals outward or inward so the holes on the surface can be fixed.
To unify the normals, please follow the steps below:
Select the mesh nodes that have holes on the surface.
Check up the Unify box.
If still the holes exist, it can be caused of that the holes are designed to be there. The faces of the hole could have been deleted before exported from the source 3D applications.
Pease apply the 2-sided feature if the problem remains.