Is the selected object a single surface? Or a polysurface. If isocurves are off, the remaining lines are the joins between surfaces.
MergeAllFaces should do the trick if they are planar.
Thanks dharman. Job done on the polysurface but not on this valid surface:
ID: 53409c61-15fd-444a-b4c6-afb95d10b6e5 (214867)
Object name: (not named)
Layer name: Default
source = from layer
index = -1
NURBS Surface"U": degree =3 CV count = 4 (0 <= U <= 219.353)
“V”: degree =3 CV count = 101 (0 <= V <= 11768.1)
4 boundary edges
Edge Tolerances: all 0
Vertex Tolerances: all 0
Render mesh: 1 mesh 297 vertices 196 polygons
Created with fast meshing parameters.
Analysis mesh: none present
Yes, I could simply separate the perimeter lines but with hundreds of files to cut this could be tedious.
Can you post that surface?
If these are flat surfaces and you are only interested in the outside border, you could just extract these as curves.