Question about smoothing issues to the model I imported a tank model from a game, extracted with Ninja Ripper. The model has some smoothing issues while even if I tried select the mesh and soft all verts, faces and edges in shading panel, the mesh still looks weird.
Also tried the auto smooth option. I'm not quite sure the faces are in this look due to only smoothing or also the flips or something inverted. With ⌃ Ctrl + N I did found some of the faces on the turret turns different while some are not.

I don't know if it has something to do with flipped normals so I recalculated the normal with ⌃ Ctrl + N or ⌃ Ctrl + ⇧ Shift + N and this is the output:
You need to do 3 things to get a clean shading:

You have sharp edges, so, in Edit mode, CtrlE &gt; Clear Sharp
You have overlapping vertices, so, Edit mode, select all and (in 2.79) W &gt; Remove Doubles
As I said, in the Properties panel &gt; Object Data &gt; Geometry &gt; Clear Custom Split Normals Data