Did you build this mesh in Rhino, or import it from someplace else? Rhino isn't great at Booleans, and working with meshes doesn't usually go as well as working with Rhino's normal NURBS surfaces. So if you built this from scratch, back up past where you created the mesh and try splitting the surface with a plane at the parting line. This should give you a negative on one side, and positive on the other.
If this is a mesh you imported (for some reason) then try Rhino's Drape function to get a surface you can deal with. If it's a symmetrical part, you should only have to drape one side, down to the parting line.
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