Smoothing makes a mess of faces

[micheus]

Any way of avoiding the unwanted edges.

That happens because the way n-gon faces are smoothed - like every boundaries vertex is connected to the face center.
Since you have a polygon with high vertices count (front and back side of the guitar body) with some peak and valley that bad topology will be generated.

So, I would recommend you to work low poly, because it's easier to control the mesh flow. You already started the object like we would expect it to be at the end.

Bellow I'm letting you just an example to help you understand why that happens and ways we can use to avoid it on smooth.

The doted green line represents how you should imagine an n-gon polygon and the way it will be subdivided on smooth. As you can see, every time we cut the n-gon face we are reducing the amount of vertices to process and changing the center for that new n-gon:


So, if you want to keep a high vertices count in a face and ensure it will be subdivided well, you need to choose good places to add edges that will keep/support the mesh in the right place:


That will resolve the issue, but still it's not a nice mesh. If you are going to use that to be 3D printed, OK - it will work. But, it's not the desirable for render or animation.

Here is an example of using the initial mesh with less density, making easier to add more details on the curve if necessary by using Edge -> Flow Connect (on the sides) and how I would define the mesh. It's not the state of the art, but I think you can get the idea.