Edit Mesh, Delete Vertex -- transparent edges? Help - Graphics

Pretty simple object, created a tube with a small thickness to it.

Did an "Edit Mesh" and then selected half of the tube and deleted the
vertices so I could have a half circle object.

Now when viewing the object, edges that were deleted are now
transparent instead of solid like the remainder of the outside of the
tube.

I read some articles that talked about selecting the vertexes you want
to seal and using the "Weld" feature, but when I attempted to do this
(in all different ways) it said "No vertices within weld threshhold."

Any tips? I'm new to this package, obviously. =)

Just looking at creating some simple objects for starters that I can
import into another game. I already have the exporter, and have been
able to export simple objects into the game. Now I want to get a
little more creative.