Strange lines after smoothing

I am quite new to Blender and need some help with a hopefully small problem.
I created a more or less complex mesh. The complete mesh is smoothed and some edges are marked as sharp with the edge split modifier.
In some faces there are kinks although the face is completely flat.
I recalculated the normals with no effect.

The hole was cut with the boolean modifier. It added some additional edges in the whole mesh. Mostly at the problematic regions. But the lines (shadow?) existed before also.

Thank you in advance!

you just need to improve your topology. try to stick with quads and uniform edge loops where you can.

What do you exactly mean with improve? Should I subdivide more faces?

No, but try to stick to quads. The errors arise from the triangles in your mesh and will cause pinching during subdivision and deformation.

there’s a sticky at the top of this subforum on edge loops, that should tell you mostly what you need to know. and the thing with tris…well, just avoid them under most circumstances.
(edit) also the quads should be even and level, not boomerang shaped or bent.

for corners you could try to add some bevels
or just try to add and edge split modifier
and see if it improves it


Thank your very much!
Now I have only with following problems.

|              |
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|       |      |
|       |      |

If I merge faces like this, they will produce the same problem like the triangles in the upper quad. Is it possible to do something like this?

That’s because you have an n-gon joined to 2 quads, if I am understanding your diagram. The pinching can be corrected, but to answer this more accurately, I would have to see more context.

Here you can see the situation.

I won’t cut all faces above to get only quads, because then I will get too many polygons.
Or is there a way to make one edge like a bezier curve?

Well if you won’t do it, you’re going to keep getting probblems. Your topology is a mess and is causing all of your problems.

Use this method for cutting holes in surfaces and be sure to use quads: