Solid vs Smooth Faces with meshes

Hi,
When I worked on the model I split it into two halves and later recombined one side with its mirror to make it look symmetrical
Now, after working on the texture and colors when rendering the model I am getting two different looks when choosing to have smooth faces vs solid faces. Below, the first image is a print screen with smooth faces, the second is with solid faces
Could anyone help me to fix the smooth?. Thank you
Peter

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difficult to say just looking like that at the pic
are you talking about the zigzag line in the middle?

did you removes all the doubles?

try to use the edgesplit modifier that may help a lot too!

Salutations

The edgesplit modifier did help. Thank you for the tip

That’s a normals issue. Ctrl+N in edit mode.