diff --git a/lib/Slic3r/TriangleMesh.pm b/lib/Slic3r/TriangleMesh.pm
index c2e543467..5aa97c9a0 100644
--- a/lib/Slic3r/TriangleMesh.pm
+++ b/lib/Slic3r/TriangleMesh.pm
@@ -563,7 +563,9 @@ sub horizontal_projection {
     }
     
     $_->make_counter_clockwise for @f;  # do this after scaling, as winding order might change while doing that
-    return union_ex(\@f, 1);
+    
+    # the offset factor was tuned using groovemount.stl
+    return union_ex([ offset(\@f, Slic3r::Geometry::scale 0.01) ], 1);
 }
 
 1;