did you try with the new Bevel nodes in 2.8
should be able to change all outside edges !
The Bevel Node is also available in experimental 2.79 builds. You can also get the edge detection behavior by doing a Vector Math(Cross Product) with Geometry Node Normal output as the top input and the Bevel Node Vector output as the bottom input. For the actual mask use the Vector Math’s Value output instead of its Vector output.
Edge nodes using bevel
happy bl