Darker lines?

[denzjos]

First af all, the picture is overexposed. If you want to correct this, you can use this easy trick :
- copy the picture to a new layer
- set the layer mode 'multiply'
- merge the two layers
   
If you want to use the picture as it is then you can try this :
- copy the picture two times to two new layers
- invert the top layer : Colours / Invert
- set the layer mode for this layer 'dodge'
- use Filters / Blur / Gaussian Blur (I suggest value 3)
- make a layer from the result : New from visible (in the layers dock)
- set this layer and the first layer (original layer) active
- set the 'new from visible' layer mode 'multiply'
If the lines are not dark enough, then copy the 'new from visible' once more (layer mode 'multiply)