Weird lighting on some faces of mesh in Blenders and in UE4 I am having a weird shading or lighting problem in a mesh.  See in the picture the faces are half-light grey and half-dark grey.

What is causing this?  When I export as an FBX and import into UE4 I get the same shading problems.  

EDIT:  It seems to be worse with smooth shading of faces, with flat shading it seems to go away, but then the model looks a bit ugly
Ok, Solved this, a combination of sharp and smooth edges is needed, so I learnt a new thing today :)