What is causing these lines?

[AdamAnt]

I need the buttons to be the same material, but im getting these lines/strokes, as you can see on the top 2 buttons... I need the grey buttons to look like the green one (no lines) just 1 color..
Even if I change the diffuse color, the grey lines are still there...

The material of the button is a multimaterial..but as far as I can see... its all the same material. Im a complete noob with materials in Keyshot..so im probably missing sometihng here...

Can anybody shed some light on this for me?

edit: i found that the button is made of 2 different parts... but even when I put the same material on both parts... you can still see difference between the 2 parts... rougness etc is also the same