After you stroke your first line dont 'scale' the selection but 'shrink' it by however many pixels you wish and do this however many times you need.
Stroking after shrinking then turning each selection into a path.
Stroking paths will be smoother.
Once you have turned selections into paths the paths will stay there in that session or if you save as a xcf. Then to select a path you just need to go to the path dialog and choose it like you would a layer.
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You may also want to check this thread out https://www.gimp-forum.net/Thread-Dispar...troke-path
