Hello!
When I use the Clip Brush to create sharp edges / borders, it seems to work.
However, when I rotate the model a bit, I can see that it created horizontal extensions which are so flat that you don’t see them if you look at the model exactely from the side, but if you rotate the model, you can see that they’re there.
How could I avoid or fix this?
That’s how the Clip brush works. It pushes/squashes the mesh rather than cutting it.
To add onto that, you’ll want to try Trim or Slice instead.
The Clip brushes do not change the topology of your model; they only push the polygons based on the stroke you apply to your model.
The Trim Curve brushes are similar to the Clip Curve brush with one fundamental difference: these brushes totally remove the polygons rather than simply pushing them toward the curve.
Lastly, the Slice Curve brush simply slices the model’s geometry and creates a different PolyGroup on each side of the drawn curve. It modifies the topology to create the slice/polygroups, but will not delete either side.