Hi all:
External Infill Pattern Concentric fails for low percentage infills (15%) , because it don't have support.
I suggest make only the last external infill in Concentric Pattern, as Slic3r do.
I am using 3 top layers, and I think that the more strong and beautiful finish can be obtained making,
a first layer 100% rectilinear (////), a second layer 100% rectilinear (\\\\\\), ( this works fine over low percentage infills ),
and then the external layer in Concentric Pattern supported by the other two previous layers.
Instead of doing all 3 top layers in concentric pattern that fails on low percentage infills.
Thanks for you work.
Regards.