How to hide the surface triangles of a Mesh so that only the outline is displayed.
key-in Facet Modify Hideedges ( Tool Setting- Minimum angle: 0 ) and except with a data point in the view
To re display the Mesh triangles, select all the Mesh elements to be affected and use the Facet Modify Hideedges Key-in with the Tool Setting ' Restore Hidden Edges ' checked.
http://communities.bentley.com/products/microstation/w/microstation__wiki/10426.repair-a-mesh-element